Human Health and Ecological Risk Assessment 

for the Use of Wildlife Damage Management Methods 

by USDA-APHIS-Wildlife Services

Chapter XXVI

The Use of Chlorophacinone in 

Wildlife Damage Management

March 2023

Peer Reviewed Final

June 2023
i

EXECUTIVE SUMMARY

Chlorophacinone is a first-generation anticoagulant rodenticide used by APHIS Wildlife Services 

(WS) to control various rodents in terrestrial environments. Chlorophacinone is registered for use 

to control rodents at rangelands, forestry areas, agricultural sites, in and around animal burrows 

and buildings (agricultural, public, industrial, commercial, residences, and food processing 

facilities), transport vehicles such as ships, trains, and aircraft, and in non-crop areas such as 

fence lines, gullies, ditches, and railroad rights-of-way, lawns, turf, golf courses, airports, and 

ornamental flower and shrub gardens. 

WS applied 4 chlorophacinone products in 6 states (Table 1) from FY11 to FY20 to control 

mountain beavers to protect standing and seedling trees, ground squirrels on airports that are

preyed upon by predators to protect aircraft that become airstrike hazards, black-tailed prairie 

dogs to protect rangelands and non-crop areas adjacent to pastures, and on airports, and pocket 

gophers to protect turf, alfalfa, and rangeland.

WS evaluated chlorophacinone's human health and ecological risk under the proposed use 

patterns. The risk to the public is minimal due to the limited volume applied annually and WS 

adherence to label requirements. Dermal exposure to workers is a risk, but the proper use of 

label-mandated personal protective equipment, adherence to label instructions, and limitations on 

the amount of product a worker may apply in one day reduce the risk of injury.

The exposure risk to terrestrial plants and aquatic environments is negligible based on the use 

pattern. WS has a single record of nontarget species take through its use of chlorophacinone bait. 

However, WS recognizes it is not possible to detect all nontarget take because animals may die 

belowground, they can be difficult to find in certain habitats, or species are wide-ranging and 

dispersed outside of the treatment area. WS monitors treatment sites and avoids treating burrows 

where they detect nontarget species. Despite this monitoring, WS recognizes nontarget species 

will be exposed to chlorophacinone bait. WS expects most target animals baited belowground 

with chlorophacinone will die in their burrows, which reduces secondary exposure potential to

nontarget species; however, some target animals will die aboveground. The Rozol Prairie Dog 

Bait label requires applicators to return to the site within 4 days after bait application and at 1- to

2-day intervals to collect and dispose of any bait or dead or dying prairie dogs and nontarget 

animals found aboveground. Despite these mitigations, poisoned animals that die aboveground 

between searches or are missed during searches remain available to scavengers. Birds are less 

sensitive to chlorophacinone than mammals, and the risk of mortality to predatory or scavenging 

birds consuming poisoned animals is low. However, birds may experience sublethal effects. WS 

finds its use of chlorophacinone will have the greatest risk to mammals that access treated 

burrows where they may ingest bait or poisoned animals. Mammalian predators and scavengers 

are potentially at risk from poisoned animals that die aboveground.
ii

Table of Contents

1 INTRODUCTION ................................................................................................................ 1

1.1 Use Pattern of Chlorophacinone .................................................................................. 2

2 PROBLEM FORMULATION................................................................................................ 5

2.1 Chemical Description................................................................................................... 5

2.2 Physical and Chemical Properties................................................................................ 5

2.3 Environmental Fate...................................................................................................... 5

2.4 Hazard Identification .................................................................................................... 6

2.4.1 Mechanism of Action and Metabolism................................................................... 6

2.4.2 Human Incidents................................................................................................... 6

2.4.3 Acute Toxicity ....................................................................................................... 7

2.4.4 Subchronic and Chronic Toxicities ........................................................................ 8

2.4.5 Developmental and Reproductive Effects ............................................................. 9

2.4.6 Neurotoxicity and Immunotoxicity Effects.............................................................. 9

2.4.7 Carcinogenicity and Mutagenicity.......................................................................... 9

2.4.8 Endocrine System Effects..................................................................................... 9

2.4.9 Toxicity of Other Ingredients ................................................................................10

2.5 Chlorophacinone Formulations ...................................................................................10

3 DOSE-RESPONSE ASSESSMENT...................................................................................11

3.1 Human Health Dose-Response Assessment ..............................................................11

3.2 Ecological Effects Analysis .........................................................................................13

3.2.1 Aquatic Effects Analysis.......................................................................................13

3.2.2 Terrestrial Effects Analysis...................................................................................13

4 EXPOSURE ASSESSMENT AND RISK CHARACTERIZATION .......................................16

4.1 Human Health Exposure and Risk Characterization....................................................16

4.1.1 Potentially Exposed Human Populations and Complete Exposure Pathways ......17

4.1.2 Exposure Evaluation............................................................................................18

4.1.3 Human Health Risk Characterization ...................................................................18

4.2 Ecological Exposure and Risk Characterization ..........................................................18

4.2.1 Aquatic Exposure Assessment and Risk Characterization ...................................19

4.2.2 Terrestrial Exposure Assessment ........................................................................20

4.2.3 Terrestrial Primary and Secondary Risk Characterization ....................................23

5 UNCERTAINTIES AND CUMULATIVE IMPACTS .............................................................25

6 LITERATURE CITED.........................................................................................................27

7 PREPARERS: WRITERS, EDITORS, AND REVIEWERS .................................................31

7.1 APHIS WS Methods Risk Assessment Committee......................................................31
iii

7.2 Internal Reviewers ......................................................................................................32

7.3 Peer Review ...............................................................................................................33

7.3.1 Peer Reviewers Selected by the Association of Fish and Wildlife Agencies.........33

7.3.2 Comments ...........................................................................................................33

List of Tables

Table 1. Annual average target animal take and chlorophacinone products applied or sold from 

FY11 to FY15 and FY16 to FY20. .............................................................................................. 4

Table 2. Acute technical grade and formulation chlorophacinone toxicity data for mammals 

(USEPA 2008). .......................................................................................................................... 8

Table 3. Registered chlorophacinone products WS applied or sold for wildlife damage 

management with their USEPA or Special Local Needs (SLN) registration number, application

rates, and target species allowed for each product. ..................................................................11

Table 4. Relevant reference studies from oral exposure. ..........................................................12

Table 5. Chlorophacinone toxicity in aquatic species. ...............................................................13

Table 6. Chlorophacinone toxicity in mammals. ........................................................................14

Table 7. Chlorophacinone toxicity in birds.................................................................................14

Table 8. Risk estimation for accidental occupational dermal exposure......................................17

Table 9. Estimated number of acres treated per day using USEPA hazard quotient benchmark for 

human health. ...........................................................................................................................18

List of Figures

Figure 1. Chemical structure for chlorophacinone. ..................................................................... 5
1

1 INTRODUCTION

Several rodent species are controlled with the rodenticide chlorophacinone, including commensal 

Old-World rodents (black rats1

, brown rats, and house mice) and several burrowing rodents such 

as mountain beavers, prairie dogs, pocket gophers, and voles. These species commonly cause 

a variety of damage to property such as residences (commensal rodents), crops, trees, and 

shrubs and can cause severe damage in a short time. The United States Department of 

Agriculture’s (USDA), Animal and Plant Health Inspection Service (APHIS), Wildlife Services 

(WS) may manage rodent populations and their damage per label instructions by placing 

chlorophacinone baits where these species feed, but in general, does not conduct commensal 

rodent control in urban areas.

Chlorophacinone was first registered as a vertebrate control agent in the United States in 1971

(USEPA 2015c). The registered uses of chlorophacinone include rodent control at agricultural 

sites (alfalfa, artichoke, barley, oats, pome, and stone fruit tree orchards, nurseries, and 

rangelands, Christmas tree, and other tree and forestry plantations), in and around animal 

burrows and buildings (agricultural, public, industrial/commercial, homes, and food processing 

facilities), transport vehicles (ships, trains, and aircraft), in non-crop areas (such as fence lines, 

gullies, ditches, and railroad rights-of-way), lawns, turf, golf courses, and ornamental flower and 

shrub gardens (USEPA 2015e). WS has used chlorophacinone products to reduce damage from 

prairie dogs, pocket gophers, ground squirrels, and mountain beavers in rangelands and adjacent 

non-crop areas, and forestry areas. WS has not used chlorophacinone products in and around 

buildings or at many other registered-use sites.

As a first-generation anticoagulant rodenticide, chlorophacinone usually requires multiple 

feedings by the target animal to deliver a lethal dose. Anticoagulants interfere with blood clotting 

and cause death due to internal bleeding generally within 5 to 7 days. Specifically, anticoagulants 

inhibit the vitamin K(1)-2,3 epoxide reductase (VKOR) enzyme and, therefore, the synthesis of 

vitamin K and clotting factors II (prothrombin), VII, IX, and X, critical components in blood clotting

(USEPA 2020b).

Chlorophacinone baits are applied in aboveground bait stations or belowground in rodent burrows 

to ensure that people and pets cannot access the baits. A mechanical broadcast bait spreader 

and spot applications can be made for voles and spot applications for ground squirrels. It is also 

available as a tracking powder, which is placed in areas inside buildings or their periphery along 

rodent runways, especially in burrows, where children, pets, and nontarget wildlife do not have 

access. The tracking powder is picked up by rodents on their fur and ingested when grooming.

WS does not use tracking powder formulations. For most WS applications, baits are placed by 

hand or through tubes from hoppers with a bait dispenser where a measured amount can be 

dropped directly into burrows. For pocket gophers, specifically, a probe is often used; applicators 

find burrow runways by pressing a probe into the ground about a foot from gopher mounds to find 

the main runway (the probe will easily drop when it hits a burrow). The probes are hollow tubes 

with a bait dispenser or side funnel that allows up to 1⁄2 cup of bait to be inserted into the burrow 

through the tube. Solid probes can be used, and the hole into the burrow from the probe is 

widened so up to 1⁄2 cup of bait can be deposited into the burrow system. The holes created by 

probes are then covered with debris and dirt while being careful not to cover the bait with dirt to 

discourage pocket gophers from plugging the hole themselves.

1 Scientific names are given in Chapter 1: Introduction to Risk Assessments for Methods Used in Wildlife Damage 

Management and not in this document except for species not in that chapter.
2

This human health and ecological risk assessment provides an evaluation of potential risks and 

hazards to human health and the environment, including nontarget animals, as a result of 

exposure to chlorophacinone from the use of chlorophacinone products by WS. The methods 

used to assess potential human health effects follow standard regulatory guidance and 

methodologies (National Research Council 1983, USEPA 2016) and align with the procedures 

and methodologies of other Federal agencies, e.g., the U.S. Environmental Protection Agency 

(USEPA 2022). The methods used to assess potential ecological risk to nontarget species

generally follow USEPA (2022) methodologies. 

This risk assessment starts with problem formulation (identifying hazards) and then evaluates 

toxicity (dose-response assessment) and exposure (identifying potentially exposed populations 

and determining potential exposure pathways for these populations). Lastly, the integration of the 

toxicity and exposure assessments provides a characterization of risks (determining if adverse 

human health or ecological risks are present and their significance). A discussion of the 

uncertainties associated with the risk assessment and cumulative effects is also included in this 

risk assessment.

1.1 Use Pattern of Chlorophacinone

WS applicators primarily apply chlorophacinone baits using hand spot baiting directly into 

subterranean burrow systems of specific burrowing rodents (to control prairie dogs, pocket 

gophers, and mountain beavers) or scattered on the ground just outside burrows (to control 

ground squirrels). Application rates vary with the density of rodent populations and target species 

being controlled. Under the Rozol Vole Bait (EPA Reg. No. 7173-242) label, WS could potentially 

use mechanical broadcasters, hand spot baiting, or bait stations to apply the bait if they used this 

product to target voles in the future. However, WS has not used this product or mechanical 

broadcast application method to date.

Hand spot baiting involves placing the bait within an active burrow opening (not for ground 

squirrels) or scattering it on trails, runways, or bare ground adjacent to burrow entrances and 

covering each placement with grass or shingle to avoid exposing nontarget organisms. For open 

burrow systems, applicators may directly hand bait or use a tube or hose from a bait dispenser 

typically mounted on the rear of an all-terrain vehicle to insert bait into burrows. Applicators may 

also use a bait dispensing probe for pocket gopher burrows. Probes are hollow tubes or metal 

rods used to locate a burrow and release bait into the burrow. The number of applications per 

pocket gopher burrow system often depends on the number of burrow mounds because baits are 

placed a foot to a yard away from each mound in the main underground runways with a probe or 

into the main burrow after the plug at the base of a mound is removed, and bait is inserted into 

the main tunnel as far as possible; the burrow is re-covered with dirt but not enough to cover the 

bait.

Many burrowing rodents taken with chlorophacinone baits will die belowground (Lee and 

Hyngstrom 2007, Nolte and Wagner 2001, Ramey et al. 2007). Thus, it is not possible to count 

the exact number of burrowing rodents taken with treatments because WS does not dig up 

burrows to determine the actual take of target or nontarget species. In the Management 

Information System (MIS) 2, WS personnel generally record the number of acres or burrows 

2 MIS - Computer-based Management Information System used for nationally tracking APHIS-WS WDM methods used 

and activities. Methods and activities are tracked such as chlorophacinone projects but take is not always estimated.
3

treated or estimate the number of target species taken with treatments. The quantity of bait 

applied is always recorded in the MIS.

If WS specialists estimated take in the MIS, that number was used when estimating average 

annual take numbers. If take was not estimated in the MIS, take numbers were estimated based 

on average occupancy of burrows by adults and young (number per burrow) or were estimated 

based on the expected number of target animals taken per pound of bait applied (number per lb.) 

or expected density (number per acre). In general, these methods likely overestimate actual take 

numbers because they assume 100% take of the target animals assumed to be present and 

exposed, which rarely occurs under actual field conditions. The following parameters and 

assumptions were used to estimate WS take with chlorophacinone baits for each target rodent 

species:

• Mountain beavers: Mountain beavers are territorial. Burrows are usually occupied by a 

single individual. Females breed once and average 3 young per year, which are weaned 

at 6 to 8 weeks of age. Young leave the burrow soon after being weaned. To estimate the 

number taken per burrow, it was assumed that 50% of burrows contained 4 mountain 

beavers (mother with 3 young) for 3 months, and there is 1 male in all of the other burrows

(this assumes a 50:50 ratio of females to males). For the remaining 9 months of the year, 

100% of burrows are occupied by just 1 mountain beaver. Therefore, this risk assessment

estimated the average number of mountain beavers taken per burrow was approximately

1.5 per burrow treated for every single 12-oz packet of bait applied. 

• Black-tailed prairie dogs: A typical prairie dog town contains groups of prairie dogs 

known as coteries. A typical coterie consists of one adult male, three or four adult females, 

and their young up to one year of age. The residents of each coterie protect their territory 

from intruders, including prairie dogs from other coteries within the town. Coteries typically

use many burrows. Black-tailed prairie dog densities range from approximately 5 to 20 

prairie dogs per acre, depending on the season. Prairie dog burrow density per acre 

ranges from approximately 10 to 50 burrows per acre in some locations. Therefore, this 

risk assessment estimated take as 1 black-tailed prairie dog per burrow treated or 12

prairie dogs per acre treated based on all their life history parameters.

• Richardson’s ground squirrels: Take of Richardson’s ground squirrel was mostly 

estimated by WS specialists in the MIS. When take was not estimated, this risk 

assessment estimated take as 10 Richardson’s ground squirrels per acre treated. 

• Plains and Knox Jones’ pocket gophers: These pocket gopher species are very 

territorial and live solitary lives except for breeding and mothers with young. They 

generally have about 4 young per litter and 1 litter per year in the north and 2 or more per 

year in the south part of their range, where they do not hibernate in the winter. WS take 

occurred in northern Texas but in areas where it was possible that they could have 1 or 2

litters per year. Young are weaned in 3–4 weeks and quickly leave the burrow after that. 

Assuming they have young in the burrow for 1.5 months per litter, have 2 litters per year, 

and are otherwise solitary, this risk assessment estimated that on average 2 pocket 

gophers would be taken per burrow treated. Pocket gophers can occur at densities up to 

8 per acre in high-density situations but this risk assessment estimated that 4 pocket 

gophers were taken per acre on average.

Overall, WS applied 4 chlorophacinone products in 6 states (Table 1) from FY11 to FY20 to control 

mountain beavers to protect standing and seedling trees, ground squirrels on airports that a 

preyed upon by predators to protect aircraft that become airstrike hazards, black-tailed prairie 
4

dogs to protect rangelands and non-crop areas adjacent to pastures, and on airports, and pocket 

gophers to protect turf, alfalfa, and rangeland. 

Table 1. Annual average target animal take and chlorophacinone products applied or sold from 

FY11 to FY15 and FY16 to FY20.

FY11–FY15 FY16–FY20

Products Applied

Species Number 

taken a

Pounds

Used

Number 

taken a

Pounds 

Used

EPA Reg. 

Number States 

Mountain Beaver 7 3.6 91 45.3 WA-060019 c WA

Black-tailed Prairie Dog 5,078 886.0 - - 7173-286 CO KS 

NM

Richardson’s Ground Squirrel b 329 66.4 - - MT-000007 c MT

Plains Pocket Gopher 185 16.5 7 0.65 7173-184 TX

Knox Jones’ Pocket Gopher 1 0.1 2 0.25 7173-184 TX

Average Annual Take (5 spp.) 5,600 972.6 100 46.2

Products Sold

Product Name Pounds Sold Pounds Sold EPA Reg. 

Number States

Rozol Prairie Dog Bait 568 1,270 7173-286 NM

Rozol Pocket Gopher Bait 141 136 7173-184 NM TX

Rozol Pocket Gopher Bait 

Burrow Builder Formula 6 - 7173-244 NM

Average Annual Pounds Sold 715 1,406 a The number of target animals taken. If the WS applicator did not estimate take, the number of targets taken was estimated. b An annual average of 1 nontarget meadow vole was taken. c SLN = Special Local Need registration

Estimates of the annual average number of target rodents killed with chlorophacinone baits by 

WS in WDM activities and the annual average amounts of chlorophacinone baits used from FY11 

to FY15 and from FY16 to FY20 throughout the United States are provided in Table 1. The only 

chlorophacinone product applied or sold by WS during FY11–FY20 was Rozol Prairie Dog Bait 

for black-tailed prairie dogs, Rozol Pocket Gopher Bait for four species of pocket gophers, Rozol 

Ground Squirrel Bait under a Special Local Need (SLN) label in Montana for Richardson’s ground 

squirrels, and Rozol Pellets under an SLN label in Washington for mountain beavers (Table 1). 

Between FY11 and FY15, WS used an average of 972.6 pounds of chlorophacinone baits to take 

an annual average of 7 mountain beavers, 5,078 black-tailed prairie dogs, 329 Richardson’s 

ground squirrels, 185 plains pocket gophers, and 1 Knox Jones’ pocket gopher. Between FY16 

and FY20, WS use decreased to an annual average of 46.2 pounds of chlorophacinone baits to 

take an annual average of 91 mountain beavers and 9 pocket gophers of two species (Table 1). 

All applications were belowground directly into the burrow except for the ground squirrels, which 

were aboveground adjacent to the burrow. These products were applied in 6 states, including

Colorado, Kansas, Montana, New Mexico, Texas, and Washington (Table 1). The only known 

nontarget take was during FY11–FY15, when WS took an annual average of 1 nontarget meadow 

vole when controlling ground squirrels at airports (Table 1).

WS also sold Rozol Prairie Dog Bait and Rozol Pocket Gopher Bait Burrow Builder Formula to 

certified pesticide applicators, given that they are RUPs (87% of total lb. sold from FY11 to FY20), 

and sold Rozol Pocket Gopher Bait as a general use pesticide (13%) (Table 1). Most 

chlorophacinone products were sold in New Mexico, with some Rozol Pocket Gopher Bait sold in 

Texas. Other than the species above, the Rozol Pocket Gopher Bait was also sold for the control 

of Northern and Botta’s pocket gophers.
5

2 PROBLEM FORMULATION

The following sections discuss the chemical description and product use, physical and chemical 

properties, environmental fate, and hazard identification for chlorophacinone. 

2.1 Chemical Description 

Chlorophacinone (2-[2-(4-chlorophenyl)-2-phenylacetyl]indan-1,3-dione; C23H15ClO3; CAS #

3691-35-8) is a pesticide active ingredient and an organic compound with a molecular weight of 

374.82 g/mol and molecular structure shown in Figure 1.

2.2 Physical and Chemical Properties

Chlorophacinone is a pale yellow microcrystalline powder (NIH 2021). It has a low vapor pressure 

of 3.58 x 10-6 torr and Henry’s law constant of 5.12 x 10-7 atm-m3

/mol suggesting a low potential 

to volatilize from water or soil into the atmosphere (USEPA 2015e;2020a). Its water solubility is

3.43 milligrams (mg)/Liter (L) at 25°C (USEPA 2015e). Chlorophacinone has an octanol/water 

partition coefficient (Kow) of 94.5 and a soil adsorption coefficient (Koc) of 20,299 milliliters

(ml)/gram (g) (USEPA 2015d).

2.3 Environmental Fate

Environmental fate describes the processes by which chlorophacinone moves and transforms in 

the environment. The environmental fate processes include 1) persistence and degradation in the 

environment, 2) mobility, and 3) migration potential to groundwater and surface water.

Chlorophacinone is slightly to moderately persistent in the environment (USEPA 2015e;2020a). 

Chlorophacinone has low vapor pressure with a low potential for dissipation through air 

volatilization (USEPA 1998b). Chlorophacinone degrades mainly through aerobic soil metabolism 

with half-lives of 17 days (sandy loam) and 47.2 days (sandy clay loam) (USEPA 2011). 

Degradation by photolysis for soil is not a significant source of degradation for belowground 

applications (SERA 2015). Chlorophacinone is hardly mobile (practically immobile) in soil (USEPA 

2015e). It binds strongly to organic materials such as the formulated baits with little available for 

leaching to groundwater or runoff to surface water (USEPA 2015d).

Figure 1. Chemical structure for chlorophacinone.
6

In water, chlorophacinone is stable at pH 7 and 9 and degrades slowly at pH 5 with a half-life of 

232 days (USEPA 2015e). Degradation in water is not expected to be a major route of degradation 

due to the negligible exposure to water resources per label restrictions (see Section 4, Exposure 

Assessment and Risk Characterization). Chlorophacinone does not bioconcentrate in aquatic 

organisms (USEPA 2020a).

2.4 Hazard Identification 

Chlorophacinone is highly toxic to mammals via oral, inhalation, and dermal exposure routes

(USEPA 2015b). The following section summarizes available acute and chronic toxicity data for 

mammals used to evaluate the hazards of chlorophacinone to human health and nontarget 

mammals.

2.4.1 Mechanism of Action and Metabolism

Chlorophacinone is a first-generation anticoagulant rodenticide (USEPA 2015b). 

Chlorophacinone is an indandione that disrupts the vitamin K cycle necessary for blood-clotting 

factors to function (USEPA 2015b). Animals exposed to chlorophacinone have increased blood 

coagulation times which can result in mortality (USEPA 2015b). Toxic symptoms include dyspnea

(labored breathing), lethargy, hemorrhage from the nose, and urethral bleeding (NIH 2021). Death 

follows excessive external and internal bleeding (USEPA 2015b).

The single oral dose evaluation in rats found chlorophacinone absorption peaked between 4 and 

6 hours with a half-life (t1⁄2) of approximately 10 hours and distributed systemically; the highest 

concentrations are found within the liver and kidneys. The blood concentration of chlorophacinone 

in rats after repeated oral doses indicates bioaccumulation. Chlorophacinone excretion over 4

days was predominately through feces (94.7–108.6% administered dose) with a minor excretion 

(<1% administered dose) through urine and respiration. A biliary excretion assay indicated 

approximately 26% of an administrated dose of chlorophacinone is excreted within eight hours 

post-exposure via the bile (USEPA 2015b).

2.4.2 Human Incidents

The USEPA reviewed human incident reports for rodenticides, including chlorophacinone, for 

incidents reported to the USEPA Office of Pesticide Programs (OPP) Incident Data System (IDS), 

the National Pesticide Information Center (NPIC), the Centers for Disease Control and 

Prevention/National Institute for Occupational Safety and Health (CDC/NIOSH), Sentinel Event 

Notification System for Occupational Risk-Pesticides (SENSOR), and Agricultural Health Study 

databases between 2004 and 2018 (USEPA 2015d). These reviews suggested that the USEPA’s 

mitigation measures to prevent accidental exposures to rodenticides that were implemented by 

USEPA in their 2008 Risk Mitigation Decision (RMD) (USEPA 2008) may have contributed to an 

overall decrease in exposure incidents involving second-generation anticoagulant rodenticides 

and incident counts for first-generation anticoagulant rodenticides remained low (USEPA 2020a). 

The 2008 RMD mitigation measures included using mandatory bait stations for all residential and 

general sales and for second-generation anticoagulant rodenticides used in outdoor aboveground

settings. Additionally, sales and distribution limits were put in place for four of the 10 rodenticides

that cause the greatest risk to wildlife. Compliance with these mitigation measures required by 

the RMD (USEPA 2008) resulted in a 65% reduction in reportable exposure incidents in children 

under 5 years of age (Gummin et al. 2020, Mowry et al. 2013). Non-occupational exposure 

incidents were expected to continue to decrease because of the completion of the phasing out of

non-compliant products in March 2015 (USEPA 2015d).
7

Between January 1, 2010, and May 27, 2015, the Main IDS reported one incident from exposure 

to chlorophacinone (an adult female suicide attempt) classified as moderate severity (USEPA 

2015d). The Aggregate IDS reported one incident involving chlorophacinone classified as having 

no or unknown effects. The SENSOR-pesticide database from 1998 to 2011 identified 12 cases 

involving chlorophacinone. Nine cases were from a single active ingredient exposure, with one

case being moderate severity and eight being low severity. The moderate severity case involved 

an insulation worker who experienced shakiness, fever, and vomiting, as well as respiratory, 

neurological, gastrointestinal, renal, and cardiovascular symptoms after touching and/or inhaling 

chlorophacinone dust while performing insulation work in a school attic. Inhalation of the product 

“dust” was the cause of eight low-severity cases. Eight of the 12 cases reported respiratory and 

neurological symptoms, and eight were occupational. One involved a homeowner who 

accidentally inhaled chlorophacinone while opening a can of gopher bait and experienced 

shortness of breath and coughing (USEPA 2015d). WS did not have any reported incidents.

The Agricultural Health Study evaluates the link between pesticide use and various health 

outcomes, including cancer, private and commercial pesticide applicators, and their spouses. 

However, commensal rodenticides are not included in the Agricultural Health Study database.

2.4.3 Acute Toxicity

Technical chlorophacinone has high acute toxicity (Toxicity Category I) via the oral, dermal, and 

inhalation exposure routes. It is not a dermal irritant, eye irritant (Toxicity Category IV), or dermal 

sensitizer (Table 2; (USEPA 2015b)).

An acute oral toxicity study to determine the lethal dose for 50% (LD50) 3 of technical 

chlorophacinone was conducted in Sprague Dawley rats4 (10 rodents/treatment group/sex). Male 

rat mortalities were observed at all treatment doses, including 0 (0/10), 2 (4/10), 3.2 (6/10), 5.2 

(4/10), 8.2 (8/10), 13.2 (10/10), 21 (9/10) mg/kilogram (kg)-body weight (bw) (deceased/total). No 

female mortalities occurred at the two lowest doses. The results were an LD50 of 3.15 mg/kg-bw 

for males, 10.95 mg/kg-bw for females, and a combined LD50 of 6.26 mg/kg-bw for this study. All 

deaths occurred four to 13 days after exposure. Chlorophacinone is a Toxicity Category I 

compound for oral exposure (USEPA 1998b).

Male New Zealand white rabbits5 were dermally exposed for 24-hrs to 0.25, 0.5, and 0.75 mg/kgbw technical chlorophacinone. Animals were observed for 21 days post-exposure. Deaths 

occurred at all doses between days five and 19, resulting in a dermal LD50 of 0.329 mg/kg-bw. 

Chlorophacinone is in the Toxicity Category I for dermal exposure (USEPA 1998b).

Groups of seven to nine Sprague Dawley rats were exposed (nasally) to 4-hour doses of 1.33, 

10.3, 11.5, or 14.5 μg/Liter (L) technical chlorophacinone. During the 21-day study monitoring

period, both male and female rats died due to non-treatment-related complications. No treatmentrelated mortalities were observed at the lowest (1.33 μg/L) dose. Mortalities accompanied by 

signs of anticoagulant activity occurred 3–8 days post-treatment, starting at 10.3 μg/L. The 

inhalation lethal concentration for 50% (LC50) of male and female rats was 7 μg/L and 12 μg/L, 

respectively, and the combined LC50 was 9.3 μg/L. Chlorophacinone is in the Toxicity Category I 

for inhalation exposure (USEPA 1998b).

3 LD50, or lethal dose, is the amount of a chemical given all at once that kills 50% of a population of test animals

4 Sprague Dawley rats are a domesticated form of the brown rat (Rattus norvegicus domestica). They are widely used 

as a laboratory rat due to their temperament and longevity but have some physiological differences from their wild 

counterpart. 5 New Zealand white rabbits (Oryctolagus cuniculus) are domesticated European rabbits that are often used for 

laboratory studies due to their quick growth, docile nature, and longevity.
8

The end-use formulations containing 0.005% chlorophacinone have low acute oral, dermal, and 

inhalation toxicity. Both formulations are mild transient eye irritants (Table 2; USEPA 2015c).

Table 2. Acute technical grade and formulation chlorophacinone toxicity data for mammals

(USEPA 2008).

Species Test Toxicity Value Toxicity 

Category

Chlorophacinone Technical Grade

Laboratory Rat Oral LD502 3.15 mg/kg-bw1 ♂; 10.95 mg/kg-bw ♀;

combined: 6.26 mg/kg-bw I

Laboratory 

Rabbit Dermal LD50 0.329 mg/kg-bw ♂ I

Laboratory Rat Inhalation LC503 7 μg/L ♂; 12 μg/L ♀; combined: 9.3 μg/L I

Laboratory 

Rabbit Eye Irritation No eye irritation IV

Laboratory 

Rabbit Dermal Irritation No skin irritation IV

Guinea Pig Dermal 

sensitization Not a skin sensitizer N/A

Rozol® Prairie Dog Bait (EPA Reg. No: 7173-286) (Liphatech 2017)

Rozol Pocket Gopher Bait (EPA Reg. No: 7173-184) (Liphatech 2007)

Laboratory Rat Oral LD50 >5,000 mg/kg-bw IV

Laboratory Rat Inhalation LC50 186 mg/L (extrapolated) IV

Laboratory 

Rabbit Dermal LD50 >2,000 mg/kg-bw III

Laboratory 

Rabbit

Eye Irritation Mild, transient irritant III

1 Milligrams (mg) per kilogram bodyweight (kg-bw). 2 LD50, or lethal dose, is the amount of a chemical given all at once that kills 50% of a population of test animals. 3 LC50, or lethal concentration, is the concentration of a chemical in the air or water that will kill 50% of the test animals when they are 

exposed to the chemical for a set period.

2.4.4 Subchronic and Chronic Toxicities

In a repeat-dose oral toxicity study, groups of 10 Sprague Dawley rats were gavaged with 

technical chlorophacinone at 0, 0.005, 0.01, 0.02, 0.04, 0.08, and 0.16 mg/kg-bw/day, seven days 

per week for 113 days. The treatment group dosed at 0.005 mg/kg-bw/day was discontinued after 

77 days due to no observed toxicity. Groups treated with 0.08 and 0.16 mg/kg-bw/day were 

terminated due to 100% mortality between treatment days 3 and 13. At 0.02 mg/kg-bw/day, four

of 10 male rats and zero of 10 female rats died between treatment days 105 and 111. At the 

conclusion of this study, the 0.01 mg/kg-bw/day group males showed a 28% increase in 

coagulation time, while the female rats showed a 6% increase. A no observable adverse effect 

level (NOAEL) of 0.005 mg/kg-bw/day and a lowest observable adverse effect level (LOAEL) of 

0.01 mg/kg-bw/day was identified from this study (USEPA 1998b).

In a 21-day dermal toxicity study, five rabbits/treatment/sex were dermally exposed to technical 

chlorophacinone 6 hours per day, five days a week at 0.08, 0.4, and 2 mg/kg-bw/day. At 2 mg/kgbw/day, 4/5 male and 1/5 female rabbits died between treatment days 14 and 21. Elevated 
9

prothrombin time was observed at 0.4 mg/kg/day. The NOAEL for this study was 0.8 mg/kgbw/day, with a LOAEL of 0.4 mg/kg-bw/day) (USEPA 1998b).

2.4.5 Developmental and Reproductive Effects

Two studies evaluated the prenatal developmental toxicity of chlorophacinone administered to 

Sprague Dawley rats and rabbits orally. Pregnant Sprague Dawley rats were orally administered 

chlorophacinone by gavage at 0, 0.0125, 0.025, 0.05, and 0.1 mg/kg-bw/day for nine days 

(gestation days 6–15). No maternal toxicity was observed at 0.05 mg/kg-bw/day (NOAEL). The 

LOAEL for maternal toxicity was 0.1 mg/kg-bw/day based on mortality. For developmental toxicity, 

treatment-related effects for developmental anomalies such as hydroureter, distended ureter, and 

total ureter anomalies were observed at all doses, and a NOAEL for developmental toxicity could 

not be determined (NOAEL <0.0125 mg/kg-bw/day, LOAEL = 0.0125 mg/kg-bw/day) (USEPA 

1998b).

In a prenatal developmental toxicity study in rabbits conducted at 0, 0.005, 0.01, 0.025, and 0.075 

mg/kg-bw/day, maternal toxicity (i.e., increased coagulation times) occurred at 0.01 mg/kgbw/day (LOAEL). In this study, the NOAEL for maternal toxicity was 0.005 mg/kg-bw/day, 10x 

lower than the LOAEL for mortality in rats. In addition, the NOAEL and LOAEL for developmental 

toxicity were lower than in the rat study, with a NOAEL of 0.01 mg/kg-bw/day and LOAEL of 0.025 

mg/kg-bw/day, but were based on the lack of sufficient fetuses/litters available for evaluation 

(USEPA 1998a;2015b). The surviving litters at treatment doses below 0.025 mg/kg-bw/day were 

not evaluated for developmental anomalies. Physical anomalies may have occurred at lower 

doses (USEPA 1998b).

2.4.6 Neurotoxicity and Immunotoxicity Effects

The Hazard and Science Policy Council (HASPOC) waived neurotoxicity studies and 

immunotoxicity studies for chlorophacinone because available studies adequately characterize 

toxicity for any quantitative risk assessments (USEPA 2015d). Chlorophacinone disrupts blood 

clotting at low doses and is unlikely to have different toxicity in repeat oral dose studies (USEPA 

2015b).

2.4.7 Carcinogenicity and Mutagenicity

A cancer risk assessment has not been conducted for chlorophacinone because long-term 

studies are not available, and chronic exposure is not likely to occur (USEPA 2015b). The 

mutagenicity studies did not indicate evidence of mutagenicity concerns for chlorophacinone

(USEPA 2015b).

2.4.8 Endocrine System Effects

USEPA developed the Endocrine Disruptor Screening Program (EDSP) to characterize endocrine 

activity in commercial products, pesticides, and environmental contaminants (USEPA 2023). The 

EDSP utilizes a two-tier risk characterization approach consisting of screening candidate 

compounds for estrogen, androgenic, and thyroid receptor activity followed by the quantification 

of impact on environmental and human health (USEPA 2023). Prior to 2012, Tier 1 screening 

involved five in vitro and six in vivo assays (Browne et al. 2015). To address the growing need for 

a more rapid but equally comprehensive review of thousands of candidate compounds, EDSP 

revised Tier 1 screening to include computational endocrine activity models and high-throughput

assays. Tier 2 testing data characterizes the endocrine-related health effects, dose response, and 

health risks of candidate compounds and substances.
10

EDSP reported chlorophacinone to have an ER agonist activity of 0 and an ER antagonist activity 

of 0.0307 with the standard of 17β estradiol agonist ER activity of 1 (USEPA 2015a). Based on 

the ToxCastTM Phase II endocrine disruption activity results, chlorophacinone was also reported 

to be active in 9 out of 21 estrogen receptor response assays and 10 out of 16 androgen receptor 

response assays. Therefore, the EDSP summarized chlorophacinone as having “significant 

estrogen and androgenic activity” (Williams et al. 2017).

2.4.9 Toxicity of Other Ingredients

Approximately 99.995% of the ingredients in the chlorophacinone products used by WS are 

“other” ingredients (Liphatech 2017;2020b). The other ingredients are considered confidential 

business information and are not included on the safety data sheets of either formulation. Thus, 

it is believed that these ingredients are likely, not toxic.

2.5 Chlorophacinone Formulations

Chlorophacinone is commercially available in multiple pesticide products registered by USEPA 

under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), including pelleted or 

whole grain (e.g., hulled oat groats) baits containing 0.005% chlorophacinone (Liphatech 

2016;2017;2018;2020a;b, USEPA 2015c;2020b). Liphatech, Inc. (Milwaukee, Wisconsin) is the 

only registrant of chlorophacinone products in the United States. 

Each chlorophacinone formulation has specific label instructions that applicators must follow 

when applying the baits. Many chlorophacinone products are restricted to underground use only

to control burrowing rodents in rangelands, croplands, forests, cropland borders, dormant 

vineyards, non-agricultural areas, and on infested bare ground areas in pastures, alfalfa, wheat, 

barley, or oat fields as specified on their labels. Some products are registered for use 

aboveground in bait stations or by mechanical broadcast equipment. Some products may be used 

in urban areas inside structures and immediately outside these buildings. Application rates on the 

labels vary with the density of rodent populations and species being controlled. Most 

chlorophacinone products are classified as restricted use pesticides (RUP), which may only be 

applied by certified pesticide applicators. WS personnel must follow the label instructions when 

applying the baits.

The chlorophacinone products used or sold by WS from FY116 to FY20 and their labeled target 

species are listed in Table 3, along with the USEPA registration number, application rates, and 

target species. Other registrations are available but were not used by WS, and WS does not plan 

to use them in wildlife damage management (WDM) at this time. The maximum concentration of 

active ingredient (a.i.) of any product that WS used was 0.005% chlorophacinone. WS used 

chlorophacinone products to control mountain beavers, prairie dogs, pocket gophers, and ground 

squirrels. 

6 FY11 denotes the federal Fiscal Year 2011, which is October 1, 2010–September 30, 2011 (the year is denoted by 

FY12, FY13, and so on).
11

Table 3. Registered chlorophacinone products WS applied or sold for wildlife damage 

management with their USEPA or Special Local Needs (SLN) registration number, application

rates, and target species allowed for each product.

a SLN Reg. No. WA-060019 was cancelled by Liphatech in 2022. WS is working with Liphatech on submitting a replacement SLN 

application to the Washington State Department of Agriculture in 2023 under the Rozol Vole Bait parent label.

b SLN product does not have a “parent” FIFRA Section 3 registration. 

c This product was not used or sold by WS between FY11–FY20. However, this product will become the parent label for the new WA 

SLN for mountain beaver in 2023.

3 DOSE-RESPONSE ASSESSMENT

3.1 Human Health Dose-Response Assessment

In the repeat-dose oral toxicity study (Section 2.4.4), 28% of male rats exhibited dose-related 

increases in coagulation times starting at 0.01 mg/kg/day compared to 6% of female rats. In the 

Product Name 

(classification)

EPA Reg. 

Number

(current 

label version 

date)

Formulation

Application Rate

(pounds or ounces of 

product applied per 

acre)

Target Animal

Rozol Pellets

(Restricted Use 

Pesticide (RUP))

SLN No. 

WA-060019

(label 

version 

expired 

12/31/2021) 

Parent label 

7173-151

(12/14/2020)

Chlorophacinone: 

0.005%

Other ingredients: 

99.995%

12 oz/burrow hole and 

maximum of 24 oz per 

burrow system per 

year;

Generally, ~3 lb/acre 

for a density of 4 

mountain 

beavers/acre

Mountain beavers

(Aplodontia rufa)

Rozol Prairie 

Dog Bait (RUP)

7173-286

(04/20/2017) 1.6–8 lb/acre

Black-tailed prairie 

dogs (Cynomys 

ludovicianus)

Rozol Pocket 

Gopher Bait

(general use 

pesticide)

7173-184 

(06/18/2007)

Up to 0.96 lb/ burrow 

system (max applied 

is ~58 lb/acre)

Pocket gophers 

(Thomomys and 

Geomys spp.)

Rozol Pocket 

Gopher Bait 

Burrow Builder 

Formula 

(RUP)

7173-244

(09/09/2009)

Make burrows the 

same depth as natural 

burrows, about 20–30 

ft apart with 6–8 

lb/acre. Can also be 

used for hand 

application.

Pocket gophers 

(Thomomys and 

Geomys spp.)

Rozol Ground 

Squirrel Oat Bait

(RUP)

SLN No. 

MT-000007

(label

version 

expires 

12/31/2025) 

b

1 tbsp. bait around 

active burrows on 

bare ground or apply 

in bait stations 20–

200 ft apart filled with 

1–4 lb bait 

Columbian ground 

squirrels (Urocitellus 

columbianus), 

Richardson’s ground 

squirrels (U. 

richardsonii)

Rozol Vole Bait c

(RUP)

7173-242

(11/03/2021) 1.5 oz/burrow Voles (Microtus spp.)
12

0.020 mg/kg-bw/day exposure group, 100% of male rats had increased coagulation times 

compared to 11% of female rats. These results, along with lower estimated acute oral and 

inhalation LD50 values for male rats compared to female rats, suggests that male mammals are 

more sensitive than female mammals to chlorophacinone. 

The repeat-dose dermal toxicity studies conducted in rabbits have yielded highly variable results,

and there is no repeat-dose inhalation toxicity data for chlorophacinone. Although 

chlorophacinone has a high dermal permeability coefficient, adverse effects from these exposure 

routes are the same as for oral exposure once chlorophacinone crosses the dermal barrier. As a 

result, USEPA (2015a) selected the developmental toxicity study in rats as the key reference 

study due to the applicable exposure route, dose regimen, treatment schedule, and relevance of 

the critical effect, which was elevated increased coagulation or prothrombin times for hazard 

characterization for the 0.005% chlorophacinone baits during registration review. USEPA 

selected the NOAEL of 0.05 mg/kg-bw/day (maternal toxicity NOAEL) as the Point of Departure 

(POD)7 for their human occupational exposure risk assessment based on elevated prothrombin 

and activated partial thromboplastin times seen at 0.01 mg/kg-bw/day. The use of this POD did 

not result in Margins of Exposure (MOE) above the Level of Concern (LOC), including a 10x 

database uncertainty factor when an oral exposure POD was used to predict toxicity from an 

inhalation route of exposure (USEPA 2016b). Furthermore, chlorophacinone has only non-food 

uses. Thus, no food tolerances are established by the USEPA (USEPA 2015b;d).

An uncertainty factor is a human health risk assessment conceptual tool based on the assumption 

that a biological response increases or decreases in relation to increasing or decreasing exposure 

to a biologically active compound. If a sufficient reduction in exposure from the first known 

threshold for adverse effects can be accomplished, then an exposure limit that is protective of 

populations, including the vulnerable, can be determined (Dankovic et al. 2015). Uncertainty 

factors are used to address the differences between experimental data and actual human health 

effects. These include uncertainties for interspecies and intraspecies differences, differences in 

duration of exposure, and incomplete or insufficient toxicity data.

Table 4. Relevant reference studies from oral exposure.

Test 

Species

Test 

Duration Critical Effect

N(L)OAEL/Uncertainty 

factors/RfD (mg/kgbw/day)Reference

Rat1 9 days Prolonged prothrombin times 0.05(0.1)/90/0.00055 (USEPA 2015b)

Rat1 9 days Prenatal developmental 

abnormalities ≤0.0125/300/0.000042 (USEPA 2015b)

Human Single 

exposure Lower prothrombin levels 0.29/100/0.00029 (Watt et al. 2005, WHO 1995)

NOAEL = No Observable Adverse Effect Level; LOAEL = Lowest Observable Adverse Effect Level; RfD – Reference 

Dose; 1Sprague Dawley Laboratory Rats

For this risk assessment, uncertainty factors of 10 and the square root of 10 (√10) were applied 

to the NOAEL to account for genetic variability between humans and the variability between 

animals and humans. To accommodate for the absence of male rodents in the reference study

(developmental toxicity study), an additional factor of √10 was applied to the uncertainty 

extrapolation for a total uncertainty factor of 100. The application of uncertainty factors to the POD 

would result in a reference dose (RfD) of 0.00055 mg/kg-bw/day (Table 4).

7 Point of Departure (POD): A data point or an estimated point that is derived from observed dose-response 

data and use to mark the beginning of extrapolation to determine risk associated with lower environmentally 

relevant human exposures.
13

Other studies are available to assess adverse effects associated with chlorophacinone use in 

study animals (Table 4). Prolonged prothrombin time (time to clotting), which is expected with 

anticoagulant exposure, has been observed in single exposure studies with humans (USEPA 

2015b, Watt et al. 2005, WHO 1995). Prenatal developmental abnormalities have also been 

observed (USEPA 2015b).

3.2 Ecological Effects Analysis

This section summarizes available chlorophacinone toxicity data for terrestrial and aquatic 

species.

3.2.1 Aquatic Effects Analysis

3.2.1.1 Aquatic Vertebrates and Invertebrates

Chlorophacinone is highly toxic to freshwater fish such as rainbow trout (LC508 is 0.452 mg/L) and 

freshwater aquatic invertebrates such as water fleas (EC50 is 0.640 mg/L) (Table 5) (USEPA 

2020a). Daphnia magna has a No Observable Adverse Effect Concentration (NOAEC) of 0.28 

mg/L (USEPA 2021a).

Table 5. Chlorophacinone toxicity in aquatic species.

Test Species Scientific Name Test LC50/EC50

(mg/L)

Reference

Bluegill Sunfish Lepomis 

macrochirus

96-hour 

LC50

0.71 As cited in (SERA 2015)

Rainbow Trout Oncorhynchus 

mykiss

96-hour 

LC50

0.452 (USEPA 2020a)

Water Flea Daphnia magna 48-hour 

EC50

0.64 (USEPA 2020a)

3.2.1.2 Aquatic Plants

Data for chlorophacinone toxicity to aquatic plants is not available. 

3.2.2 Terrestrial Effects Analysis

3.2.2.1 Mammals and Birds

Chlorophacinone has very high acute oral and dietary toxicity in mammals (Table 6) (USEPA 

2020a). In rats, the acute oral toxicity LD509 is 0.8 mg ai/kg-bw, and the acute dietary LC50 is 1.14 

mg ai/kg-diet (USEPA 2020a). The developmental NOAEL for mammals is 0.010 mg/kg-bw/day, 

based on a 2-generation reproduction study in rabbits, using the lack of sufficient fetuses at the 

end of the study as the endpoint (USEPA 2020a).

8 LC50, or lethal concentration, is the concentration of a chemical in the air or water that will kill 50% of the 

test animals when they are exposed to the chemical for a set period. 9 LD50, or lethal dose, is the amount of a chemical given all at once that kills 50% of a population of test 

animals
14

Table 6. Chlorophacinone toxicity in mammals.

Test species Test Reference

Black-tailed Prairie Dog LD50 = 1.94 mg/kg-bw, oral gavage, single dose (USEPA 2011)

Brown Rat (lab)

LC50 = 1.14 mg/kg-diet, 5-day dietary exposure (USEPA 2011)

Acute oral LD50 = 0.8 mg/kg-bw, doses of 0.16 

mg/kg-bw for five days to equal LD50 dose (USEPA 2020a)

Domestic Rabbit

Developmental NOAEL = 10 μg/kg-bw/day, oral 

gavage, daily from days 7 to 19 of gestation (USEPA 2011)

Acute oral LD50 = 50 mg/kg-bw (WHO 1995)

Target and nontarget mammals that ingest chlorophacinone baits may take several days to die 

from toxicity (e.g., mountain beavers fed 5 g of 0.005% chlorophacinone bait daily died within 15 

days (Arjo et al., 2004)). Due to this delay in symptoms, animals may continue to eat available 

bait, ingesting more than a sufficient dose to cause toxicity. After a single oral dose of 0.336 mg, 

chlorophacinone had a blood half-life of 11.7 days and a liver half-life of 35.4 days in the mouse 

(USEPA 2020a). Chlorophacinone has a blood half-life of 0.4 days in rats that received a single

dose of 4–5 mg/kg-bw (USEPA 2020a) and a liver half-life of 5.9 days in black-tailed prairie dogs 

that received a single dose of 23 mg (USEPA 2020a, Witmer et al. 2016).

USEPA (2011) summarized laboratory studies on the secondary hazard of chlorophacinone to 

mongoose, coyote, red fox, weasel, and European ferret that were fed one or more poisoned prey 

for one to 90 days. Prey were poisoned with 0.0025%–0.01% chlorophacinone bait. Across these

studies, 27 (49%) of 55 test mammals died after feeding on poisoned rodents (USEPA 2015e).

In birds, chlorophacinone is moderately toxic based on an acute oral basis (acute oral LD50 of 258 

mg/kg-bw in northern bobwhite and is highly toxic based on a subacute dietary basis (subacute 

dietary LC50 of 56 mg/kg-diet in northern bobwhite) (Table 7) (USEPA 2020a). The red-winged 

blackbird has an acute oral LD50 of 430 mg/kg-bw, and the mallard has an acute dietary LC50 of 

172 mg/kg-diet (USEPA 2004). In chronic exposure studies, chlorophacinone has a NOAEC of 

0.046 mg/kg-diet and a Lowest Observable Adverse Effect Concentration (LOAEC) of 0.096 

mg/kg-diet, based on a reduction in mean body weight of survivors in a 14-day reproductive 

toxicity study in mallard ducks (USEPA 2020a). The standardized acute toxicity studies conducted 

for first-generation anticoagulant rodenticides such as chlorophacinone may make these 

rodenticides appear less toxic than they are (Vyas and Rattner 2012). The time course of action 

for first-generation anticoagulant rodenticides requires large dosages in acute toxicity studies 

resulting in high LD50 values. However, several studies on birds and mammals have found these 

animals are more sensitive to first-generation anticoagulant rodenticides given at low dosages 

over several days than in the standardized acute oral toxicity test (Vyas and Rattner 2012).

Table 7. Chlorophacinone toxicity in birds.

Test species Test Reference

Northern Bobwhite 

(Colinus virginianus)

Acute oral LD50 = 258 mg/kg-bw (USEPA 2020a)

Acute dietary LC50 = 56 mg/kg-diet (USEPA 2020a)

Red-winged Blackbird Acute oral LD50 = 430 mg/kg-bw (USEPA 2004)

Mallard (Anas 

platyrhynchos)

Acute dietary LC50 = 172 mg/kg-diet (USEPA 2004)

NOAEC1 = 0.046 mg/kg-diet (USEPA 2020a)

LOAEC2 = 0.096 mg/kg-diet (USEPA 2020a) 1 NOAEC = No Observable Adverse Effect Concentration

2 LOAEC = Lowest Observable Adverse Effect Concentration

USEPA (2011) summarized laboratory studies on the secondary hazard of chlorophacinone to 

carnivorous birds that were fed one or more poisoned prey for 3 to 61 days. Prey were poisoned 
15

with 0.005%–0.01% chlorophacinone bait. No deaths were recorded in the secondary bird 

exposure studies that included 106 carnivorous birds from nine species (barn owl, great horned 

owl, tawny owl (Strix aluco), red-tailed hawk, American kestrel, Eurasian buzzard (Buteo buteo), 

carrion crow (Corvus corone), black-billed magpie, and white stork (Ciconia ciconia). Some birds 

exhibited symptoms of chlorophacinone toxicity, such as external bleeding (n = 20), internal 

hematoma (n = 20), and increased blood coagulation time (n = 28) by the end of the study duration

(USEPA 2015e).

Between 1990 and 2008, 54 wildlife incident reports of chlorophacinone exposure were reported 

in the Incident Data System maintained by the USEPA. Of these incidents, 8 were detectable 

chlorophacinone residues in live animals. Of the remaining incidents involving carcasses, 21 were 

classified as “highly probable,” 14 as “probable,” and 11 as “possible” chlorophacinone exposure 

(USEPA 2020a). Incidents involving avian species exposed to chlorophacinone included geese, 

quail, wild turkeys, barn owls, turkey vultures, bald eagles, and red-tailed hawks. Mammals 

included squirrels, raccoons, wild boars, badgers, black bears, coyotes, and bobcats (USEPA 

2020a). Federally listed and protected species included the bald eagle, golden eagle, and San 

Joaquin kit fox (Vulpes macrotis mutica). Some of these exposures occurred after broadcast 

application aboveground, where baits and target animals are more accessible to nontarget 

species. Belowground applications reduce some exposure to nontarget species. These incidents 

did not involve WS applications. 

Between 2014 and 2018, the California Department of Pesticide Regulation tested animals for 

anticoagulant residues (CDPR 2018, USEPA 2020a). Chlorophacinone was detected in 12 out of 

152 animals tested. The species tested were not provided, nor was the application method (e.g., 

aboveground broadcast application versus belowground application). 

USEPA (2004) queried the American Society for Prevention of Cruelty to Animals (ASPCA) 

Poison Control Center from November 1, 2001, to June 16, 2003, for pet exposures, primarily 

dogs, to chlorophacinone. There were 42 cases involving chlorophacinone exposure out of 2,334 

cases involving rodenticides. These exposures did not involve WS applications.

WS has found relatively few nontarget species in areas where chlorophacinone bait had been 

applied. From FY11 to FY20, five dead nontarget meadow voles were found at one application. It 

is probable that other nontarget species were taken because WS does not dig up burrows, but 

the species likely taken are not listed as threatened or endangered species because WS does 

not treat in areas where those could be impacted.

3.2.2.2 Reptiles and Terrestrial Phase Amphibians

Acute, subacute, and chronic toxicity information is lacking for reptiles and amphibians. Birds are 

surrogate species for terrestrial amphibians and reptiles, indicating chlorophacinone has 

moderate acute oral toxicity and is highly toxic based on a subacute dietary basis in these species.

3.2.2.3 Terrestrial Invertebrates and Microorganisms

In redworms (Eisenia fetida), the subchronic LC50 is >1000 mg/kg-soil, and the NOAEC for 

mortality is 309 mg/kg-soil (USEPA 2011). Decreased body weights (sublethal effect) were

observed at 95 mg/kg-soil.

In a study on the reproductive effects of chlorophacinone on the burying beetle (Nicrophorus 

orbicollis), larvae were fed for 5–10 days on rat carcasses poisoned with 0.005% chlorophacinone 

bait (the residue amount in the carcasses was not provided) and adults were fed 3 mg/kg-diet 
16

chlorophacinone contaminated ground beef for 28 days with no effects on reproduction but there 

was observed decrease in larval emergence (USEPA 2011).

Information on the effects of chlorophacinone on microorganisms is limited. USFS (2015)

summarized two studies on the exposure of bacteria to chlorophacinone, where no adverse 

effects were observed.

3.2.2.4 Terrestrial Plants

There is a lack of information on chlorophacinone toxicity in terrestrial plants. USEPA (USEPA 

2004;2020a) finds the exposure and risks to terrestrial plants are likely to be minimal.

4 EXPOSURE ASSESSMENT AND RISK CHARACTERIZATION

The exposure assessment for chlorophacinone begins with an assessment of human health and 

ecological exposure pathways for chlorophacinone. For human health exposure, a complete 

exposure pathway includes (1) a release from a source (an application of chlorophacinone bait), 

(2) an exposure point where human contact can occur, and (3) an exposure route such as 

ingestion, inhalation, or dermal contact where contact can occur (USEPA 2019).

For ecological exposures, WS applies chlorophacinone baits directly into burrows or tunnels 

mechanically or by hand and aboveground adjacent to the burrow for the ground squirrels. 

Between fiscal years 2016 and 2020, all WS applications were by hand directly to burrows. The 

exposure assessment focuses on both belowground and aboveground applications for ecological 

receptors.

Exposures for relevant human and ecological populations are then quantitatively evaluated for 

each pathway, accounting for factors such as the frequency and duration of exposure, the toxicity 

of the chemical, and the sensitivity of the exposed population.

Risk is the likelihood of harm from a specific hazard and exposure to that hazard. This section

also provides quantitative and qualitative assessments of human health and ecological risks 

associated with WS’s uses of chlorophacinone products. The evaluation of documented 

chlorophacinone health exposure data and relevant animal exposure studies can quantify the risk 

of impact on human health and non-target fish and wildlife if accidentally exposed. Deterministic 

methods are used, where appropriate, to determine if expected environmental residues exceed 

toxicity data suggesting possible risk. In other cases, a more qualitative discussion regarding risk 

may be used, relying on literature and additional information to further elaborate on the potential 

for injury or harm.

The results of the exposure assessment and risk characterization can be used to inform risk 

management decisions and to develop strategies to minimize exposures and potential harm to 

human and ecological health.

4.1 Human Health Exposure and Risk Characterization

This section discusses the potential exposure pathways for chlorophacinone on human health. It

examines the populations that are most at risk for exposure, the various methods of application, 

and the personal protective equipment used to minimize exposure. It also evaluates the potential 

for accidental exposure and assesses the associated risks. Finally, it provides a risk 
17

characterization for WS’s use of chlorophacinone products based on the available data on human 

health.

4.1.1 Potentially Exposed Human Populations and Complete Exposure Pathways

Based on the expected WS use pattern for chlorophacinone applications, workers who apply 

rodenticide baits are the subpopulation with the highest risk of exposure. Exposure during 

transportation is not anticipated because the material is in sealed containers. The baits are ready 

to use off the shelf with no mixing required. The application methods for WS use of 

chlorophacinone pelleted baits include bait stations, burrow baiting, and aboveground treatments 

or in or beside the infested burrow (no less than six inches into active brown or black rat burrows)

(Liphatech 2020b). Mechanical broadcast bait spreading and aboveground spot applications can 

also be made for voles. Rozol Prairie Dog Bait (treated grain bait) can be applied by hand scoop 

or a mechanical bait application machine. It may only be used in underground applications 

between October 1st and March 15th (Liphatech 2017). The mechanical bait application machine 

is designed, constructed, and operated to ensure that bait is placed at least six inches into active 

prairie dog burrows. For bait stations specifically, units must be secured or otherwise immobilized

if bait can be shaken from stations when lifted.

Following label directions, including the use of proper personal protective equipment (PPE), will 

minimize worker exposure to chlorophacinone baits via inhalation and dermal contact routes when 

manually or mechanically applying chlorophacinone bait. Waterproof gloves are required for any 

person who applies chlorophacinone baits as well as any WS personnel who retrieves carcasses 

or unused bait following application. Additional required PPE includes a long-sleeved shirt, long 

pants, shoes, and socks. Rozol Prairie Dog Bait and Rozol Vole Bait are restricted use pesticides

that can only be used by certified applicators or persons under their direct supervision and only 

for those uses covered by the certified applicator’s certification.

Although the potential for exposure from the proposed WS use pattern is low, accidental exposure 

may occur during application. Accidental occupational exposure for workers was further quantified 

for the dermal contact route associated with the bait formulations. Ingestion routes are not 

considered in this exposure assessment because the formulations used by WS are either treated 

grain, pellets, or packets of pellets, and the risk of incidental ingestion is extremely low. The risk 

of exposure inhalation is deemed to be minimal due to outdoor (open air) only use of labelmandated application restrictions and PPE.

As per label requirements, chlorophacinone products used aboveground, such as Rozol Pellets 

(Liphatech 2020b), are only allowed for use in locations out of reach to children, pets, domestic 

animals, and nontarget wildlife or in tamper-resistant bait stations. Tamper-resistant bait stations 

prevent access to bait compartments and must be used if treatment areas are accessible to 

children and pets. Tamper-resistant packaging also prevents unauthorized post-application 

exposure for adults or the public. Any bait spilled during treatment must be collected to reduce 

the risk of nontarget animal exposure. Stronger bait stations are needed in areas open to hoofed 

livestock, raccoons, bears, or other potentially destructive animals and places prone to vandalism. 

As a result of these use restrictions, the public is not considered a vulnerable population for direct 

exposure to chlorophacinone utilized by WS.

The potential exposure to chlorophacinone associated with belowground applications, such as 

Rozol Prairie Dog Bait or Rozol Pocket Gopher Bait, is minimal for the public. Although oral 

exposure to all formulations of chlorophacinone is hazardous, label restrictions and the use of 

prepackaged bait suggest that dietary exposure is most always an incomplete exposure pathway. 
18

Chlorophacinone has a low potential for volatilization due to its low vapor pressure. It will adsorb 

readily to organic material in the soil, minimizing inhalation risks and leaching into nearby water 

bodies or aquifers. In addition, label restrictions state that no applications are allowed directly to 

water, areas where surface water is present, or intertidal areas below the mean high-water mark

(Liphatech 2019). As a result, surface and groundwater exposure pathways are also incomplete.

4.1.2 Exposure Evaluation

This section quantitatively evaluates worker exposures from an accidental, direct dermal contact 

pathway while applying baits. Under the accidental exposure scenario, it is assumed that the 

applicator’s chemical-resistant gloves are broken. This exposure evaluation scenario considers 

potential exposures to pelleted formulations. Bait block formulations have not been evaluated due 

to the negligible risk of accidental dermal exposure.

The estimates of exposure to certified applicators are based on surrogate study data available in 

the Pesticide Handlers Exposure Database (PHED) (USEPA 2021b). The “trap/bait station 

refillable” or “applicator granules by hand” exposure scenario in the PHED table is the closest 

scenario representing potential exposure for the application method on the label for Rozol Pellets 

and sachets containing 0.005% chlorophacinone baits. Under this exposure scenario, the 

exposure unit for dermal contact under the single-layer (gloves, long-sleeve shirt, long pants, 

shoes plus socks) and double-layer PPE is 71.38 and 40.68 mg/lb. of active ingredient (lb. ai) 

applied, respectively. The exposure unit for a scenario where a WS applicator lacks single-layer 

PPE or wears inadequate hand protection, such as broken gloves, is 104 mg/lb. ai applied.

Lower and upper application use rates were estimated from pocket gopher and prairie dog 

ecological studies (Colorado State University Extension 2011, Miller 1950, Oklahoma 

Cooperative Extension Service 2017) and label-mandated treatments (Liphatech 2017). For 

example, pocket gopher density per acre can range from 16 to 60 individual burrows in some 

areas (Andelt and Case 2016, Baldwin 2019, Oklahoma Cooperative Extension Service 2017). 

The Rozol Pocket Gopher Bait label has a maximum application rate of 4.8 x 10-5 lb. ai per burrow 

(Liphatech 2007). Thus, the maximum amount of chlorophacinone per acre in pocket gopher 

applications would be 0.000768–0.00288 lb. (350– 1270 mg). Prairie dog density per acre ranges 

from approximately 10 to 50 burrows per acre in some locations (Colorado State University 

Extension 2011, National Park Service 2021). The Rozol Prairie Dog Bait label has a maximum 

application rate of 8 x 10-6 lb. ai lb. ai per burrow (Liphatech 2017). Thus, the maximum amount 

of chlorophacinone per acre would be 0.00008–0.0004 lb. (36–181 mg)) in prairie dog 

applications. 

The other relevant, conservative assumptions used in this exposure evaluation included an 

assumption that an applicator weighs 70 kg, spends 30 minutes per pesticide treatment 

application, and there is accidental dermal contact with 1% of the bait applied when PPE integrity 

is compromised without replacement for 8 hours (USEPA 2016). The following equations are used 

to estimate the exposure dose of direct contact for workers:

Exposure Dose = Daily Dose Rate/Body Weight 

Daily Dose Rate = Unit Exposure (mg/lb ai) x Application Rate (lb ai/acre) x Area Treated 

(acre/day) x Dermal Absorption Rate (%)

Exposure doses were estimated for the application rates specified on the labels and summarized 

in Table 8.
17

Table 8. Risk estimation for accidental occupational dermal exposure.

Parameter Units Values Values Values Source

‡No gloves/damaged PPE1 ‡Single layer gloves ‡Double layer PPE1

PDR1=UE1*AR1*A1*ABS1

UE1 applicator 

granules by hand mg/lb ai 104 71 40.3 (USEPA 2021b)

AR1 lb ai/acre A/D1 AR1 lb ai/acre A/D1 AR1 lb ai/acre A/D1

Mountain Beaver SLN Labels

Prepackaged 

sachets

12 oz.

sachet 0.00113 33 0.00113 48 0.00113 85 Calculated on WS use

Rozol Prairie Dog Bait Label 

†High AR1 8 lb/acre 0.0004 >100 0.0004 >100 0.0004 >100 Calculated 

Low AR1 1.6 lb based on label /acre 0.00008 >100 0.00008 >100 0.00008 >100

Rozol Pocket Gopher Bait Label 

High AR1 58 lb/acre 0.00288 12 0.00288 17 0.00288 30 Calculated 

Low AR based on label 1 15 lb/acre 0.00077 43.8 0.00077 64 0.00077 >100

ABS1 % 0.01 0.01 0.01 (USEPA 2007)

PDR1 mg/day 0.035 0.035 0.035 Calculated

Dose = PDR/BW

BW1 kg 70 70 70 (USEPA 2007)

Dose mg/kgbw/day0.00055 0.00055 0.00055 Calculated

*Hazard Quotient = Dose/RfD

RfD1 mg/kgbw/day 0.00055 0.00055 0.00055 ^NOAEL1 of 0.05 

mg/kg-bw/day Hazard Quotient 1 1 1 1 PPE = Personal Protective Equipment; UE = Unit Exposure; AR = Application Rate; A/D = Acres/Day; ABS = % Material Absorbed Dermally; PDR = Potential Daily Rate; BW = Body 

Weight; RfD = Reference Dose; NOAEL = No Observable Adverse Effect Level

* The ratio of the potential exposure to the substance and the level at which no adverse effects are expected. If the Hazard Quotient is calculated to be less than 1, then no adverse 

health effects are expected as a result of exposure. If the Hazard Quotient is greater than 1, then adverse health effects are possible. The Hazard Quotient cannot be translated to a 

probability that adverse health effects will occur and is unlikely to be proportional to risk. It is especially important to note that a Hazard Quotient exceeding 1 does not necessarily mean 

that adverse effects will occur.

† Application density and treatment levels are based on label restrictions.

‡ Based on hand application estimates of exposure by Pesticide handler exposure database.

^ Based on a NOAEL of 0.05 mg/kg-bw/day from the developmental toxicity study in rats as the Point of Departure (see Section 3.1).
18

4.1.3 Human Health Risk Characterization 

This section discusses the quantitative characterization of risks for adverse human health effects 

associated with potential accidental occupational exposure to chlorophacinone baits for WS use 

of more potentially hazardous “loose” bait products, Rozol Prairie Dog Bait and Rozol Pocket 

Gopher Bait. The volume of chlorophacinone baits applied annually is limited, and personnel 

follow label requirements, resulting in minimal risks to the public.

To quantify the human health risks to applicators associated with accidental occupational dermal 

exposure to chlorophacinone, a hazard quotient (HQ) was calculated using the USEPA risk 

estimation equation for non-carcinogens. The maximum number of acres a certified applicator 

could apply 0.005% chlorophacinone baits under the proposed WS uses without an increased 

risk was calculated using a hazard quotient of one. The calculated acres a WS employee could 

safely work per day are summarized in Table 9.

Table 9. Estimated number of acres treated per day using USEPA hazard quotient benchmark for 

human health.

Personal Protection 

Equipment1

Maximum Number of Acres to be Treated Per Day3

Rozol Prairie Dog Bait (7173-286) Rozol Pocket Gopher Bait (7173-184)

*Low AR2,3

(10 

placements/acre)

High AR2,3

(50 

placements/acre)

Low AR2,4

(16 

placements/acre)

High AR2,4

(60 

placements/acre)

No or Broken Gloves >100 >100 44 12

Single-layer PPE >100 >100 67 17

Double-layer PPE >100 >100 >100 30 1 Pesticide Handlers Exposure Database; single layer = gloves, long sleeve shirt, long pants, shoes, and socks; double layer = single 

layer PPE + coveralls 

2 Hazard quotient of 1

3 Calculated from Rozol Prairie Dog Bait label, and (Colorado State University Extension 2011), 4 Calculated from Rozol Pocket Gopher Bait label, and (Miller 1950, Oklahoma Cooperative Extension Service 2017).

HQs above a value of 1 suggest an increased risk to applicators based on exposure assumptions. 

A single applicator should apply chlorophacinone baits to no more than 27 and 83 acres per day 

for high-density and low-density applications, respectively. The risks of injury to chlorophacinone 

rodenticides are low when label instructions are followed, and there is minimal risk of 

chlorophacinone-induced injury for applicators. To maintain minimal to no risks of injury for 

applicators, individual applicators should not exceed the recommended maximum acres treated 

per day.

In summary, the limited volume of chlorophacinone baits applied annually, and personnel 

adherence to label requirements result in minimal risks to the public. The calculated acres a WS 

employee could safely work per day are summarized in Table 9, and individual applicators should 

not exceed the recommended maximum acres treated per day to maintain minimal to no risks of 

injury.

4.2 Ecological Exposure and Risk Characterization

Risk characterization combines information from the dose-response assessment with the 

exposure assessment to determine the potential adverse effects on aquatic and terrestrial 

species. USEPA refers to this ratio as the risk quotient and compares this to pre-established acute 

and chronic Levels of Concern (LOC). USEPA (USEPA 2011;2020b) characterized risks from 
19

direct chlorophacinone bait consumption and consumption of chlorophacinone-contaminated 

carcasses for aquatic and terrestrial vertebrates and invertebrates. In their risk characterization, 

USEPA used broadcast applications in the exposure scenario. In this risk assessment, WS uses 

USEPA’s risk evaluations (USEPA 2011;2020b), peer-reviewed scientific literature, product 

labels, and WS use patterns and take data (Table 1) to characterize the ecological risks 

associated with WS applications of chlorophacinone bait.

4.2.1 Aquatic Exposure Assessment and Risk Characterization

The potential for aquatic exposure to chlorophacinone bait products used by WS was evaluated. 

Table 3 shows the types of chlorophacinone bait products that WS could use. Applicators are not 

allowed to apply these products to water or areas where surface water is present or to intertidal 

areas below the mean high-water mark. Rozol Pocket Gopher Bait and Rozol Prairie Dog Bait are 

applied belowground, and although the Rozol Pellets label allows some aboveground use, the 

label restricts aboveground outdoor use to bait stations. WS did not use chlorophacinone baits 

aboveground from FY11 to FY20, but it could use Rozol Pellets for aboveground applications in 

orchards to control voles in the future. However, the prior 10-year use pattern suggests that almost 

all future applications will be belowground.

An accidental spill of chlorophacinone bait next to or into a water body could result in exposure to 

aquatic species. WS does not expect accidental water exposure to occur based on its use 

patterns. In the unlikely event that there is an accidental spill, it is unlikely that a hazardous amount 

of chlorophacinone would end up in the water. USEPA queried its Incident Data System (IDS) for 

reports of chlorophacinone exposure to nontarget species between 1990 and 2008 and found no 

reports for aquatic species.

USEPA (2015e) modeled aquatic exposure for chlorophacinone and found the estimated 

environmental concentrations for the peak concentration, 21-day average concentration, and 60-

day average concentration were orders of magnitude lower than the aquatic toxicity endpoints. 

USEPA (2015e) found the likelihood of aquatic impacts from potential exposure is insignificant. 

WS does not anticipate aquatic exposure through its use of chlorophacinone. WS treatments are 

primarily belowground. Chlorophacinone is not mobile in soils and binds strongly to the other 

ingredients in the bait formulations. These characteristics indicate chlorophacinone will not leach

from the baits or burrows into groundwater or run off into surface waters (USEPA 2015d).

The exposure assessment found aquatic exposure to be negligible based on WS use pattern and 

label restrictions, as well as findings from USEPA’s ecological risk assessment (USEPA 2020a).

The USDA Forest Service (USFS) conducted a risk assessment on the belowground application 

of chlorophacinone bait to control black-tailed prairie dogs and found aquatic exposure to be 

negligible (SERA 2015). 

USEPA estimated aquatic exposure potential (summarized in section 4.2.1) and found aquatic 

taxa are not at risk from chlorophacinone (USEPA 2020a). USEPA used the highest modeled 

exposure in their estimate, which was broadcast application of chlorophacinone in an orchard; 

USEPA’s analysis included chronic exposure to freshwater fish and freshwater invertebrates 

(Daphnia), acute exposure to green algae, and acute and chronic exposure to saltwater fish and 

Mysid shrimp (USEPA 2020a). Based on USEPA risk estimates from aquatic exposure, label 

restrictions, and WS typical use pattern, which involves direct application to burrows by hand, 

APHIS considers the risk to aquatic species negligible. Even for broadcast applications in 

orchards, the risk is considered minimal.
20

4.2.2 Terrestrial Exposure Assessment

Exposure of nontarget terrestrial animals to chlorophacinone may be directly through eating bait 

(primary exposure) or by preying on or scavenging animals poisoned with chlorophacinone

(secondary exposure). Exposure to aquatic sources is not expected; thus, drinking contaminated 

water is not an exposure pathway considered in this assessment.

The labels for the chlorophacinone products that WS may use provide instructions to reduce 

nontarget exposure to chlorophacinone. The Rozol Vole Bait label instructs applicators using 

hand-baiting to place bait in the hole, trail, or runway and to cover the bait with grass or shingle 

to avoid exposing nontarget organisms or to place the bait in a tamper-resistant bait station and 

not to apply where raptors are actively feeding on voles (Liphatech 2020a). These measures

would reduce some exposure to nontarget predators and scavenger species. The Rozol Prairie 

Dog Bait label, Rozol Pocket Gopher Bait label, and the Rozol Pellets SLN label to control 

mountain beavers allow for belowground use only (Liphatech 2007;2016;2017;2018). The Rozol 

Prairie Dog Bait label limits applications to October through March, when the target animal is most 

likely to consume the bait. Similarly, the Rozol Pellets SLN label limits applications to October 

through February. The restricted time reduces exposure to some animals, particularly those that 

are not in the area or active during that time. The Rozol Prairie Dog Bait label also instructs 

applicators to remove bait that spills aboveground or is placed less than six inches down the 

burrow entrance and requires the applicator to return to the site within four days after bait 

application, and at one to two day intervals to collect and dispose of any bait or dead or dying 

prairie dogs found on the surface. This search must continue for at least two weeks, but longer if 

carcasses are still found at that time. The removal of aboveground carcasses of poisoned animals 

reduces secondary exposure to predators and scavengers.

4.2.2.1 Primary Exposure

Between FY11 and FY20, all WS applications were belowground (Table 1). Direct exposure of 

nontarget species to treated bait from these applications is mostly limited to those animals that 

can access the burrow. However, WS expects a percentage of bait to be moved aboveground as 

animals enter and exit the burrow (USEPA 2009). 

Small granivorous mammals that share a treated burrow with the target animal are likely to ingest 

a toxic amount of chlorophacinone bait. USEPA (2004) summarized a study where 0.01% and 

0.005% chlorophacinone baits were used to treat California ground squirrels in rangelands. Bait 

was applied by spot baiting or in bait stations. For the nontarget mice (deer mice, San Joaquin 

pocket mice (Perognathus inornatus), and woodrats) found dead, at least 86% of the mortality 

was likely due to primary exposure to baits.

Birds that are herbivorous or insectivorous are not expected to be attracted to chlorophacinone 

baits, which are grain-based. Exposure of birds to underground applications of bait is expected to 

be minimal but not likely zero (USEPA 1998b). Two wild adult turkey deaths were attributed to 

chlorophacinone poisoning, likely from primary exposure to bait applied within a prairie dog colony

but not applied by WS (Ruder et al. 2011). Grain baits were found in the crop and ventriculus of 

both turkeys, and chlorophacinone was detected in the livers. Applications to prairie dog burrows 

are supposed to be directly into the burrow and not applied at the surface. Access by the turkeys

indicates that either the chlorophacinone bait was close to the surface due to the angle of the

burrow entrance, that bait was spilled aboveground (Ruder et al. 2011), or bait was moved to the 

surface through animal movement in and out of the burrow (USEPA 2009). Any of these 

alternatives could have resulted in enough bait reaching the surface for the turkeys to ingest a 
21

toxic dose. A dead western meadowlark, which is a ground-foraging songbird, was also found in 

a prairie dog colony that was treated with chlorophacinone bait. The bird had signs of 

hemorrhaging in the brain and pectoral muscle tissue and chlorophacinone residues of 0.59 and 

0.49 μg/g in the liver and intestinal contents, respectively, indicating chlorophacinone exposure

(Vyas et al. 2013). WS was not involved in this incident either.

Exposure of terrestrial and soil-dwelling invertebrates could occur should they come into direct 

contact with chlorophacinone bait in burrows. Insects may be attracted to and feed on baits placed 

in burrows and tunnels and subsequently be consumed by insectivorous wildlife. Chlorophacinone 

is immobile in soil and is not expected to leach into the soil, indicating a localized effect. WS’ use 

pattern should not result in widespread exposure to invertebrates. Chlorophacinone is minimally 

toxic to redworms (Eisenia fetida) with a 14-day LC50 greater than 300 mg a.i./kg soil (USEPA 

2020a). In a laboratory experiment in France, residue levels detected in gray garden slugs 

(Deroceras reticulatum) exposed to 5 g of 0.005% chlorophacinone pat bait (not available in the 

U.S.) between one and five days was 0.6 to 3.3 mg/kg tissue (Alomar et al. 2018). The authors 

estimated the elimination half-life in slugs as four days. The slugs did not display adverse effects 

in the study. This study suggests that terrestrial mollusks that ingest chlorophacinone bait could 

be a source of secondary exposure to species that feed on mollusks. 

Exposure to terrestrial plants is negligible based on WS use patterns.

4.2.2.2 Secondary Exposure

Secondary exposure could occur for predators and scavengers that can access burrows with 

poisoned animals or if they kill or scavenge poisoned animals aboveground. Mammalian 

carnivores and scavengers are particularly at mortality risk from chlorophacinone, although 

sublethal effects in birds may result in decreased survival or reproduction (see section 3.2.2 for a 

review of secondary exposure studies). It can take four to 10 or more days for a target animal to 

die after ingesting a toxic dose of chlorophacinone bait (USEPA 2020a, Vyas et al. 2012). In one 

laboratory study, the average time to death for prairie dogs that consumed, on average, 47 g of 

0.005% chlorophacinone bait was 15 days (Witmer et al. 2016). During this time, the animal 

continued to feed and move about. Researchers have observed predators and scavengers 

hunting treated areas more than nearby untreated areas (Vyas et al. 2012, Vyas et al. 2017), 

likely because poisoned target animals become lethargic prior to death and are easier to capture

(USEPA 2020a).

In a field study, Vyas et al. (2017) compared the number of visits by ferruginous hawks to blacktailed prairie dog colonies treated with Rozol Prairie Dog Bait (0.005% chlorophacinone) to control

(untreated) colonies. They found the hawks hunted in the treated colonies only, likely because 

the poisoned prairie dogs were lethargic and not as aware of their surroundings, making them 

easier to capture. In a field study across 10 treated prairie dog towns, 10 animal carcasses, 9 of 

which were prairie dogs, were found aboveground (Lee and Hyngstrom 2007, USEPA 2009). 

Eight of the carcasses were intact, but two had been scavenged. The rate of prairie dog carcasses 

was one per 14 acres searched. The researchers also noted that they saw five impaired prairie 

dogs 10 days or more after bait applications.

In another field study evaluating secondary poisoning from chlorophacinone bait applications to 

control Richardson’s ground squirrels, the chlorophacinone bait was placed in the burrow 

openings, which were then left open. A second application was made 48 hours later. An American 

badger and 3 long-tailed weasels died nine days after these chlorophacinone bait applications 

and had signs of bleeding and hemorrhages (Proulx 2010). In a study conducted in Kansas (Lee 
22

and Hyngstrom 2007), nine prairie dogs and one cottontail rabbit were found dead aboveground 

out of 11,479 treated burrows illustrating the minimal potential for belowground treatments to 

potentially have an impact on non-fossorial animals.

A target animal typically must feed multiple times on bait to receive a lethal dose of 

chlorophacinone. The accumulation of chlorophacinone residues in an animal depends on the 

amount of bait the animal eats, how close together the feedings are, how long since the prior

consumption of chlorophacinone, and its metabolism rate of chlorophacinone. 

One study found the greatest risk of secondary exposure to chlorophacinone residues in prairie 

dogs that fed on an average of 47 g over two days of 0.005% Rozol Prairie Dog Bait was within 

14 days of application (Witmer et al. 2016). Residues were highest three to six days after 

exposure. The half-life of chlorophacinone residues in the prairie dogs was 5.9 days for liver tissue

and 6.3 days for whole bodies (minus the liver). At the average of 15 days to death, 

chlorophacinone residues in prairie dogs declined, reducing the risk of secondary poisoning 

(Witmer et al. 2016). In the Witmer et al. (2016) study, the two-day exposure to chlorophacinone 

baits likely underestimates the intake and residue levels from field applications, where bait is 

available to animals for more than two days. 

Vyas et al. (2012) also found a decline of hepatic (liver) chlorophacinone residues over time in 

black-tailed prairie dogs and thirteen-lined ground squirrels. Vyas et al. (2012) identified several 

factors that may have caused this decline, including the short hepatic half-life of chlorophacinone, 

poisoned animals that may stop feeding on bait as they develop symptoms, and reduced 

availability of bait over time. In a study on the efficacy of 0.01% chlorophacinone bait for Belding’s

ground squirrels, 38 whole squirrel carcasses were located aboveground (mostly near burrow 

entrances), and 86% had detectable levels of chlorophacinone (Ramey et al. 2007). The mean 

level of chlorophacinone in the whole body of ground squirrel carcasses was 0.16 mg/kg tissue,

and in the liver, 0.13 mg/kg tissue. Carcasses were predominately found between 24 and 48

hours after one bait application and between 48 and 96 hours after a second application that 

occurred two days later. The authors summarized that a few (2.4%) of the poisoned ground 

squirrels died aboveground, and only some of these carcasses posed a secondary hazard to 

scavengers depending on the tissues consumed. USEPA (2004) summarized several additional 

studies measuring whole-carcass chlorophacinone residue levels in ground squirrels, mice, rats, 

voles, and pocket gophers fed 0.005–0.01% chlorophacinone baits. In ground squirrels given 

either 0.005% or 0.01% bait in the field (number of exposure days was unknown), the wholecarcass residue levels were 0.57 and 1.27 mg/kg tissue, respectively. In the laboratory, rats given 

0.005% bait for 5 days has a whole-carcass residue level of 0.47 mg/kg tissue. Voles fed 0.005%

bait in the laboratory for less than 9 days had an average whole-carcass residue level of 3.2 mg/kg 

tissue. House mice had whole-carcass residue levels of 6.0 mg/kg tissue after being fed 0.0075%

bait for three days.

The Rozol Prairie Dog Bait label requires applicators to return to the treatment site at 1- to 2-day

intervals after baiting to collect and dispose of any bait or dead or dying prairie dogs found on the 

surface (Liphatech 2017). The search for dead or dying prairie dogs and other nontarget animals 

must continue for at least 2 weeks or longer if carcasses are still being found. The removal of 

dying or dead animals would reduce some secondary exposure. Follow-up visits take time and 

resources to conduct, and possible non-compliance with label requirements intended to reduce 

secondary exposure would negate the benefits (Vyas 2013). However, WS applicators are trained 

to comply with label requirements, and WS does not expect non-compliance with the label’s

secondary exposure mitigation measures.
23

A review of select secondary exposure laboratory studies is in section 3.2.2. Mammals are 

sensitive to secondary exposure to chlorophacinone. Across the laboratory studies on secondary 

exposure USEPA (2015e) evaluated, 27 (49%) of the 55 test mammals died after feeding on 

poisoned rodents. In the studies summarized in section 3.2.2, exposure of carnivorous birds to 

poisoned prey did not result in bird deaths (USEPA 2015e); however, sublethal effects in birds 

may result in decreased survival and reproduction (Rattner et al. 2015). A study in kestrels 

examined secondary exposure mortality and sublethal effects in kestrels that were fed for 21-days 

on mice that consumed an average of 1.14 mg ai chlorophacinone. No kestrels died; however,

sublethal effects in the form of hemorrhaging were reported (Radvanyi et al. 1988). Localized 

bleeding outside of blood vessels (hematoma) was observed on the lungs, liver, heart, and 

pectoral muscles of exposed birds. In another study, kestrels were fed tissue from rats fed Rozol 

bait and similarly had evidence of hemorrhage most frequently observed in the pectoral muscle 

and heart (Rattner et al. 2015), along with other sublethal effects. Based on the sublethal 

responses in kestrels, the authors found environmentally realistic concentrations of 

chlorophacinone (e.g., reported chlorophacinone concentrations found in small mammals 

following field baiting trials) could affect the survival of free-ranging raptors (Rattner et al. 2015). 

4.2.3 Terrestrial Primary and Secondary Risk Characterization

It can take several days of consecutive feedings to deliver a lethal dose of chlorophacinone to the 

target animal, as well as nontarget species (USEPA 2020a). Similarly, secondary exposure to 

nontarget species would usually require that carnivores and scavengers consume multiple prey 

items poisoned with chlorophacinone to receive a lethal dose. The lethal exposure of nontarget 

terrestrial species is reduced but not eliminated by belowground applications. Most target animals 

die belowground (Lee and Hyngstrom 2007, Primus et al. 2001, USEPA 2009), reducing 

secondary exposure potential. 

APHIS expects minimal risk to terrestrial plants based on lack of exposure through WS use pattern 

and chlorophacinone’s mode of action. Birds are less sensitive than mammals to 

chlorophacinone, but some adverse effects, including sublethal effects, are possible through 

direct and secondary exposure. Small granivorous mammals that can access belowground

treatment sites could consume enough bait to receive a toxic dose. Terrestrial invertebrates can 

experience sublethal effects from chlorophacinone bait exposure, as demonstrated by larval 

survival effects for burying beetles (Nicrophorus orbicollis), weight change in redworms (USEPA 

2011;2020a), and chlorophacinone residues detected in a terrestrial mollusk (Alomar et al. 2018).

USEPA (2020a) calculated the single-day and 6-day primary bait exposure risk quotients for 

Passeriformes birds (songbirds) and rodents at 3 weight classes. USEPA calculated risk quotients 

by dividing an estimated environmental concentration from broadcast applications by a toxicity 

endpoint. Only rodents in the 15- and 35-kg weight classes had risk quotients that exceeded the 

LOC after a 1-day exposure, and all 3 mammal weight classes had risk quotients that exceeded 

LOCs after a 6-day exposure. The LOCs for acute and chronic risks to vertebrates are 0.5 and 

1.0 and are meant to be protective of community-level effects. USEPA found direct consumption 

of bait by bird species is unlikely to lead to toxic exposure, but rodents and other mammals that 

consume enough bait are at risk of exposure. USEPA (2020a) risk estimation using broadcast 

applications exceeds that for WS use patterns, which are predominantly belowground

applications. In an evaluation of exposure risks, USEPA (1998b) found minimal exposure to birds 

is anticipated from underground applications. APHIS emphasizes that although the exposure risk 

to birds is minimal and it is unlikely that granivorous birds would consume a toxic dose of 

chlorophacinone bait administered belowground, the risk of toxicity from the consumption of bait 

is not negligible ((Ruder et al. 2011, Vyas et al. 2013), see Section 4.2.2). Birds that are mainly 
24

herbivorous or insectivorous are not expected to be at risk from primary exposure to 

chlorophacinone bait. However, secondary risks could also occur for nontarget species that rely 

on terrestrial invertebrates during some portion of their lifecycle.

In a field study on secondary exposure to predators and scavengers, chlorophacinone residues 

were measured in the liver and carcasses of Belding’s ground squirrels, and voles (Microtus sp.) 

collected aboveground and Botta’s pocket gophers collected underground (Primus et al. 2001).

Field applications were made by spot baiting or bait stations. The maximum residue levels (0.82 

μg/g Belding squirrel and 4.1 μg/g voles) were then used to assess secondary toxicity to birds 

and mammals in 3 weight classes using USEPA’s standard evaluation procedures to calculate a 

risk quotient, as described above. Similar to the findings in the USEPA evaluation (USEPA 

2020a), there is little risk of secondary toxicity for avian species, but the acute risk is predicted for 

all three weight classes of mammals.

USEPA (2004) conducted a comparative risk evaluation of several rodenticides and concluded 

primary risks of broadcast-applied chlorophacinone to birds were low to moderate and secondary 

risks were low. Using birds as surrogate species, consuming chlorophacinone bait by terrestrial 

reptiles and amphibians would have the same primary and secondary risks as birds. In mammals, 

primary and secondary risks were evaluated as high (USEPA 2004). WS finds the risks 

associated with WS’ use of chlorophacinone bait will be lower than the USEPA estimates (USEPA 

2004;2020a) due to primarily conducting applications belowground.

A predator or scavenger feeding on a single animal poisoned with chlorophacinone may not result 

in death; the predator or scavenger would likely need to feed on several poisoned animals to 

cause sublethal effects and mortality. USEPA (2004) summarized several laboratory studies on 

the secondary hazards of chlorophacinone to predatory and scavenging birds. The lack of 

mortality in these laboratory studies in birds may have been due to the carcasses they were fed 

having low residue levels due to the relatively rapid metabolism and excretion of chlorophacinone. 

For example, in rats given 1.4 mg chlorophacinone, about 90% of chlorophacinone was 

metabolized or excreted within 2 days (unpublished study cited in (Primus et al. 2001)).

The reduction in available prey in an area due to chlorophacinone treatment could impact food 

availability for predators. WS anticipates predators would extend their hunting range into new 

areas to accommodate for the loss of prey in any given area. 

WS maintains records of nontarget species take in WDM, including the use of chlorophacinone 

baits. WS has no records of nontarget take through its use of chlorophacinone bait for FY16–

FY20 but took an annual average of one meadow vole during FY11–FY15. WS recognizes it is 

not possible to detect all nontarget take because animals mostly die belowground, may be difficult 

to detect in certain habitats, or are wide-ranging and dispersed outside the treatment area after 

exposure. WS monitors proposed treatment sites and avoids treating burrows where they detect 

nontarget species. Despite this monitoring, WS recognizes nontarget species that can access 

treated burrows may be exposed to chlorophacinone bait. In a study using a different 

anticoagulant (brodifacoum), 90% of plains pocket gophers died belowground (as cited in (Primus 

et al. 2001)). In a study using chlorophacinone, the baiting of 10 prairie dog towns produced only 

10 carcasses, 9 that were prairie dogs, which were found aboveground (about 1 carcass per 14 

acres) (Lee and Hyngstrom 2007, USEPA 2009). WS expects most of the target animals it baits 

belowground with chlorophacinone will die underground, reducing exposure to scavenging 

nontarget species; however, some die aboveground, but typically very few. The Rozol Prairie Dog 

Bait label instructs applicators to return to the site within 4 days after bait application and at 1- to

2-day intervals to collect and dispose of any bait or dead or dying prairie dogs found on the 
25

surface. Despite these mitigations, poisoned animals may die aboveground between searches or 

are missed during searches and remain available to predators and scavengers. Birds are not as 

sensitive to chlorophacinone as mammals, and the risk of mortality to predatory and scavenging

birds that consume poisoned animals is lower compared to nontarget mammals. However, birds 

feeding on poisoned animals may experience sublethal effects. WS finds the use of 

chlorophacinone will have the greatest risk to mammals that access treated burrows where they 

may ingest bait or poisoned animals. Some risk to mammalian predators and scavengers occurs 

from eating poisoned animals that die aboveground, with the most risk for those that can feed on 

animals that died in their burrows.

5 UNCERTAINTIES AND CUMULATIVE IMPACTS

The uncertainties associated with this risk evaluation arise primarily from a lack of toxicity 

information for all chlorophacinone formulations. USEPA reviews and Safety Data Sheets for 

chlorophacinone products provide some toxicity values that can be used to make general 

comparisons between the formulations. However, variations in toxicity study design, formulations,

and active ingredient concentration limit this. Uncertainties related to chronic and sublethal effects 

data for some fish and wildlife, as well as surrogacy of test organisms, are typical for most 

pesticides; however, field data related to nontarget chlorophacinone poisoning of wildlife is more 

abundant.

Conservative assumptions of exposure, behavior, and human variability are used to develop 

estimates of impacts on the environment and human health. Using these conservative 

assumptions, when label-mandated application instructions and PPE procedures are followed, 

impacts on human health and the environment should be minor. Individuals with impaired liver 

function may be at greater risk than other members of the population because chlorophacinone 

is metabolized by liver mixed-function oxidases (i.e., the cytochrome P450 enzyme system) (NIH 

2021).

In a study investigating in-utero (unborn offspring) exposure in rats, researchers found

malformations in unborn rat offspring. A NOAEL of chlorophacinone exposure to unborn SpragueDawley rats could not be established. Based on this study, APHIS suggests that WS-certified

applicators who are pregnant do not work with chlorophacinone and avoid areas treated with 

chlorophacinone.

Another area of potential uncertainty in this risk assessment is the potential for cumulative impacts 

on human health and the environment from the use of chlorophacinone in the WS program. Areas

where cumulative impacts could occur are 1) repeated worker and environmental exposures to 

chlorophacinone from program application, 2) co-exposure to other chemicals with a similar mode 

of action, and 3) exposures to other chemicals affecting the toxicity of chlorophacinone. The Rozol 

Prairie Dog Bait label does not allow the use of other rodenticides containing anticoagulants in 

prairie dog towns during the treatment period allowed on the label (Liphatech 2017). Repeated 

exposures that could lead to increased risks of injury from accidental chlorophacinone exposure 

by WS applications are expected to be minimal due to strict WS applicator adherence to labelrequired PPE. 

Due to WS' use of anticoagulant rodenticides in addition to chlorophacinone in its program to 

control rodents, WS personnel could potentially be exposed to multiple compounds of similar 

mechanisms of action. Since the Rozol Prairie Dog Bait label does not allow other rodenticides 

containing anticoagulants such as diphacinone to be used during the treatment period, co-
26

exposure to a single applicator during the application of chlorophacinone baits is likely reduced, 

although WS personnel could possibly treat one area with chlorophacinone and another with 

another anticoagulant. However, when the products are applied according to label requirements, 

workers' exposure and aggregate risks should be minimal. 

Human health effects and environmental impacts of chlorophacinone in combination with other 

pesticide chemicals are not well understood. As a result, product label use requirements should 

be strictly followed, including the proper use of PPE when loading, applying, or handling 

chlorophacinone-containing products. The risk of public exposure to chlorophacinone should be 

minimal.

Chlorophacinone baits that are not consumed are retrieved from the field by WS applicators for 

certain application methods. Any unrecoverable baits left in the environment from WS use will 

begin to biodegrade, with a half-life of 47 days in soil (USEPA 2015e), which minimizes human 

risks and nontarget take.
27

6 LITERATURE CITED

Alomar, H., A. Chabert, M. Coeurdassier, D. Vey, and P. Berny. 2018. Accumulation of anticoagulant 

rodenticides (chlorophacinone, bromadiolone and brodifacoum) in a non-target invertebrate, the 

slug, Deroceras reticulatum Science of the Total Environment 610-611:576-582. 

https://www.sciencedirect.com/science/article/pii/S0048969717321150.

Andelt, W.F., and R.M. Case. 2016. Managing Pocket Gophers. Colorado State University Extension, 

Natural Resources Series.

Baldwin, R. 2019. UC IPM Pest Notes: Pocket Gophers. UC ANR Publication 7433. University of California, 

Agriculture and Natural Resources, Integrated Pest Management Program.

Browne, P., R.S. Judson, W.M. Casey, N.C. Kleinstreuer, and R.S. Thomas. 2015. Screening Chemicals for 

Estrogen Receptor Bioactivity Using a Computational Model. Environ Sci Technol 49:8804-8814. 

https://pubs.acs.org/doi/pdf/10.1021/acs.est.5b02641.

CDPR. 2018. An investigation of anticoagulant rodenticide data submitted to the Department of Pesticide 

Regulation.

Colorado State University Extension. 2011. Managing Prairie Dogs, Natural Resources Series: Wildlife.

Dankovic, D.A., B.D. Naumann, A. Maier, M.L. Dourson, and L.S. Levy. 2015. The Scientific Basis of 

Uncertainty Factors Used in Setting Occupational Exposure Limits. Journal of Occupational and 

Environmental Hygiene 12:S55-68. doi:10.1080/15459624.2015.1060325.

Gummin, D.D., J.B. Mowry, M.C. Beuhler, D.A. Spyker, D.E. Brooks, K.W. Dibert, L.J. Rivers, N.P.T. Pham, 

and M.L. Ryan. 2020. 2019 Annual Report of the American Association of Poison Control Centers’ 

National Poison Data System (NPDS): 37th Annual Report. Clinical Toxicology 58:1360-1541. 

doi:10.1080/15563650.2020.1834219.

Lee, C.D., and S.E. Hyngstrom. 2007. Field efficacy and hazards of Rozol bait for controlling black-tailed 

prairie dogs. Unpublished Report.

Liphatech. 2007. Rozol Pocket Gopher Bait. EPA Registration Number 7173-184.

_____. 2016. Rozol Pellets for Control of Mountain Beaver in Forestry Plantations (SLN No. OR-060026 

and SLN No. WA-060019). Liphatech Inc.

_____. 2017. Rozol Prairie Dog Bait. EPA Registration Number 7173-286.

_____. 2018. Rozol Technical Bulletin for Management of Mountain Beavers in Forestry Plantations.

_____. 2019. Safety Data Sheet for Rozol Pellets EPA Registration Number 7173-151. Milwaukee, WI.

_____. 2020a. Rozol Vole Bait. EPA Registration Number 7173-242.

_____. 2020b. Rozol Pellets Label, EPA Reg. No. 7173-151, December 14, 2020. 

Miller, M. 1950. Eradication of pocket gophers: Comparative field tests demonstrate best poisons, baits, 

and dosages for practical gopher control. Hilgardia 4:8-10. 10.3733/ca.v004n12p8.

Mowry, J.B., D.A. Spyker, L.R. Cantilena, J.E. Bailey, and M. Ford. 2013. 2012 Annual Report of the 

American Association of Poison Control Centers’ National Poison Data System (NPDS): 30th 

Annual Report. Clinical Toxicology 51:949-1229. doi:10.3109/15563650.2013.863906.

National Park Service. 2021. Black-tailed Prairie Dogs. https://www.nps.gov/articles/000/black-tailedprairie-dogs.htm accessed 03/14/2023.

National Research Council. 1983. Risk assessment in the Federal government: managing the process. 

National Academy Press, Washington, DC.

NIH. 2021. PubChem CID 19402: Chlorophacinone National Institutes of Health, National Library of 

Medicine, National Center for Biotechnology Information.

https://pubchem.ncbi.nlm.nih.gov/compound/19402 accessed 03/14/2023.
28

Nolte, D.L., and K. Wagner. 2001. Non-Target Impacts of Strychnine Baiting to Reduce Pocket Gopher 

Populations on Forest Lands in the United States. Pages 59-70 in H. J. Pelz, D. P. Cowan, and C. J. 

Feare, eds. Advances in vertebrate pest management, II. Filander Verlag, Furth, Germany. 

https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1603&context=icwdm_usdanwrc.

Oklahoma Cooperative Extension Service. 2017. Controlling Pocket Gophers ID: NREM-9001. 

https://extension.okstate.edu/fact-sheets/controlling-pocket-gophers.html accessed

03/14/2023.

Primus, T.M., J.D. Eisemann, G.H. Matschke, C. Ramey, and J.J. Johnston. 2001. Chlorophacinone Residues 

in Rangeland Rodents: An Assessment of the Potential Risk of Secondary Toxicity to Scavengers. 

Pages 164-180 in J. J.J., editor. Pesticides and Wildlife. ACS Symposium Series 771. American 

Chemical Society, Washington D.C., USA.

Proulx, G. 2010. Field evidence of non-target and secondary poisoning by strychnine and chlorophacinone 

used to control Richardson’s ground squirrels in southwest Saskatchewan. Proceedings of the 

Prairie Conservation and Endangered Species Conference 9:128-134.

Radvanyi, A., P. Weaver, C. Massari, D. Bird, and E. Broughton. 1988. Effects of chlorophacinone on captive 

kestrels Bulletin of Environmental Contamination and Toxicology 41:441-448. 

https://link.springer.com/content/pdf/10.1007/BF01688891.pdf.

Ramey, C.A., G.H. Matschke, and R.M. Engeman. 2007. Chlorophacinone baiting for Belding’s ground 

squirrels. Proceedings of the Wildlife Damage Management Conference 12:526-537.

Rattner, B.A., K.E. Horak, R.S. Lazarus, S.L. Schultz, S. Knowles, B.G. Abbo, and S.F. Volker. 2015. Toxicity 

reference values for chlorophacinone and their application for assessing anticoagulant 

rodenticide risks to raptors. Ecotoxicology 24:720-734.

Ruder, M.G., R.H. Poppenga, J.A. Bryan, M. Bain, J. Pitman, and M.K. Keel. 2011. Intoxication of nontarget 

wildlife with rodenticides in northwestern Kansas. Journal of Wildlife Diseases 47:212-216.

SERA. 2015. Chlorophacinone: Human Health and Ecological Risk Assessment, Final Report [TR-056-10-

03b] USDA Forest Service Contract: AG-3187-C-12-0009. 145 pp.

USEPA. 1998a. Appendix E HED Toxicity Profile for Chlorophacinone. 

https://www3.epa.gov/pesticides/endanger/litstatus/effects/redlegfrog/2011/chlorophacinone/appendix-e.pdf accessed 03/14/2023.

_____. 1998b. Reregistration Eligibility Decision (RED) - Rodenticide Cluster, EPA738-R-98-007. 

https://nepis.epa.gov/Exe/ZyPDF.cgi/20000OOQ.PDF?Dockey=20000OOQ.PDF accessed

03/14/2023.

_____. 2004. Potential risks of nine rodenticides to birds and nontarget mammals: a comparative 

approach. https://www.regulations.gov/document/EPA-HQ-OPP-2006-0955-0005 accessed

03/14/2023.

_____. 2007. Dermal Exposure Assessment: A summary of EPA approach (EPA 600/R-07/040F). National 

Center for Environmental Assessment Office of Research and Development, Washington D.C.

_____. 2008. Risk Mitigation Decision for Ten Rodenticides. https://www.regulations.gov/document/EPAHQ-OPP-2006-0955-0753 accessed 03/14/2023.

_____. 2009. IRB Efficacy Review: Rozol Prairie Dog Bait. 

https://www3.epa.gov/pesticides/chem_search/cleared_reviews/csr_PC-067707_18-Mar09_a.pdf accessed 03/14/2023.

_____. 2011. Risks of chlorophacinone use to the federally threatened Alameda Whipsnake (Masticophis 

lateralis euryxanthus), California Tiger Salamander (Ambystoma californiense), Central California 

Distinct Population Segment, And the Federally Endangered California Tiger Salamander 

(Ambystoma californiense) Sonoma County Distinct Population Segment and Santa Barbara 

County Distinct Population Segment, Salt Marsh Harvest Mouse (Reithrodontomys raviventris), 
29

and San Joaquin Kit Fox (Vulpes macrotis mutica). U.S. Environmental Protection Agency, Office 

of Pesticide Programs, Environmental Fate and Effects Division June 29, 2011.

_____. 2015a. Endocrine disruption, Endocrine Disrupter Screening Program (EDSP) Estrogen Receptor 

Bioactivity. https://www.epa.gov/endocrine-disruption/endocrine-disruptor-screening-programedsp-estrogen-receptor-bioactivity accessed 03/14/2023.

_____. 2015b. Memorandum – Chlorophacinone. Human Health Scoping Document in Support of 

Registration Review (EPA-HQ-OPP-2015-0778-0005). U.S. Environmental Protection Agency.

_____. 2015c. Memorandum – BEAD Chemical Profile for Registration Review: Chlorophacinone (067707), 

EPA-HQ-OPP-2015-0778-0006. U.S. Environmental Protection Agency.

_____. 2015d. Memorandum – Rodenticides: Summary of Hazard and Science Policy Council Meeting on 

October 1, 2015: Recommendations on Data Requirements for Rodenticides, 10 pp, EPA-HQ-OPP2015-0769-0008. U.S. Environmental Protection Agency.

_____. 2015e. Registration Review-Preliminary Problem Formulation for Ecological Risk and 

Environmental Fate, Endangered Species, and Drinking Water Assessments for Chlorophacinone 

(EPA-HQ-OPP-2015-0778-004). U.S. Environmental Protection Agency.

_____. 2016. Overview of Risk Assessment in the Pesticide Program. https://www.epa.gov/pesticidescience-and-assessing-pesticide-risks/overview-risk-assessment-pesticide-program accessed

03/14/2023.

_____. 2019. Guidelines for Human Exposure Assessment EPA/100/B-19/001. U.S. Environmental 

Protection Agency.

_____. 2020a. Memorandum - Seven Anticoagulant Rodenticides: Draft Ecological Risk Assessment for 

Registration Review (EPA-HQ-OPP-2015-0481-0052). U.S. Environmental Protection Agency, 

Office of Chemical Safety and Pollution Prevention.

_____. 2020b. Chlorophacinone, Diphacinone and its Sodium Salt, Brodifacoum, Bromadiolone, 

Difenacoum, Difethialone; Draft Human Health Risk Assessment for Registration Review of 

Anticoagulant Rodenticides (EPA-HQ-OPP-2015-0768-0043). U.S. Environmental Protection 

Agency.

_____. 2021a. Records on Chlorophacinone. U.S. EPA ECOTOX Knowledgebase. U.S. Environmental 

Protection Agency.

_____. 2021b. Occupational Pesticide Handler Unit Exposure Surrogate Reference Table. 

https://www.epa.gov/sites/default/files/2021-05/documents/occupational-pesticide-handlerunit-exposure-surrogate-reference-table-may-2021.pdf accessed 03/14/2023.

_____. 2022. Overview of Risk Assessment in the Pesticide Program. 

https://www.epa.gov/risk/conducting-human-health-risk-assessment accessed January 26, 

2023.

_____. 2023. Endocrine Disruptor Screening Program (EDSP) Overview. 

https://www.epa.gov/pesticides/epa-rebuilds-endocrine-disruptor-screening-programsoliciting-public-comment-new accessed 03/14/2023.

Vyas, N.B., C.S. Hulse, and C.P. Rice. 2012. Chlorophacinone residues in mammalian prey at a black-tailed 

prairie dog colony. Environmental Toxicology and Chemistry 31:2513-2516. 

https://setac.onlinelibrary.wiley.com/doi/full/10.1002/etc.1968.

Vyas, N.B., and B.A. Rattner. 2012. Critique on the use of the standardized avian acute oral toxicity test 

for first generation anticoagulant rodenticides. Human and Ecological Risk Assessment: An 

International Journal 18:5:1069-1077.

Vyas, N.B. 2013. Untested pesticide mitigation requirements: ecological, agricultural, and legal 

implications. Drake Journal of Agricultural Law 18.2:335-348.

Vyas, N.B., C.S. Hulse, C.U. Meteyer, and C.P. Rice. 2013. Evidence of songbird intoxication from Rozol 

application at a black-tailed prairie dog colony. Journal of Fish and Wildlife Management 4:97-
30

103. https://meridian.allenpress.com/jfwm/article/4/1/97/138770/Evidence-of-SongbirdIntoxication-From-RozolR.

Vyas, N.B., F. Kuncir, and C.C. Clinton. 2017. Influence of poisoned prey on foraging behavior of 

ferruginous hawks. The American Midland Naturalist 177:75-83. https://bioone.org/journals/theamerican-midland-naturalist/volume-177/issue-1/0003-0031-177.1.75/Influence-of-PoisonedPrey-on-Foraging-Behavior-of-Ferruginous-Hawks/10.1674/0003-0031-177.1.75.full.

Watt, B.E., A.T. Proudfoot, S.M. Bradberry, and J.A. Vale. 2005. Anticoagulant Rodenticides. Toxicological 

Reviews 24:259-269. doi:10.2165/00139709-200524040-00005.

WHO. 1995. Anticoagulant rodenticides. https://apps.who.int/iris/handle/10665/37676 accessed

03/14/2023.

Williams, A.J., C.M. Grulke, J. Edwards, A.D. McEachran, K. Mansouri, N.C. Baker, G. Patlewicz, I. Shah, J.F. 

Wambaugh, R.S. Judson, and A.M. Richard. 2017. The CompTox Chemistry Dashboard: a 

community data resource for environmental chemistry. Journal of Cheminformatics 9. 

https://doi.org/10.1186/s13321-017-0247-6.

Witmer, G.W., N.P. Snow, and R.S. Moulton. 2016. Retention time of chlorophacinone in black-tailed 

prairie dogs informs secondary hazards from a prairie dog rodenticide bait. Pest Management 

Science 74:725-730. https://onlinelibrary.wiley.com/doi/full/10.1002/ps.4045.
31

7 PREPARERS: WRITERS, EDITORS, AND REVIEWERS 

7.1 APHIS WS Methods Risk Assessment Committee

Writers for “Use of Chlorophacinone in Wildlife Damage Management Risk Assessment”:

Primary Writer: Andrea Lemay 

Position: USDA-APHIS-Policy and Program Development (PPD), Environmental and Risk 

Analysis Services (ERAS), Biological Scientist, Raleigh, NC

Education: BS Plant and Soil Science (Biotechnology) - University of Massachusetts; MS Plant 

Pathology -North Carolina State University 

Experience: Thirteen years of service in APHIS conducting risk analysis. Four years of 

experience in preparing environmental analyses in compliance with the National 

Environmental Policy Act.

Primary Writer: Michael McCaskill

Position: USDA-APHIS-Policy and Program Development (PPD), Environmental and Risk 

Analysis Services (ERAS), Environmental Health Specialist, Riverdale, MD

Education: B.S. Environmental Science – University of Florida; MPH Industrial HygieneUniversity of South Carolina, Ph.D. Toxicology-Florida Agriculture and Mechanical University

Experience: Ten years of experience conducting human toxicological research at Florida 

Agriculture and Mechanical University, University of Nebraska Medical Center, and Tulane 

University. Four years of experience conducting human health and environmental 

toxicological risk assessments and assisting environmental compliance programs at the 

Florida Department of Health, Commonwealth of Pennsylvania, and USDA.

Primary Writer: Jim Warren 

Position: USDA-APHIS-Policy and Program Development (PPD), Environmental and Risk 

Analysis Services (ERAS), Environmental Toxicologist, Little Rock, AR 

Education: B.S. Forest Ecology and M.S. Entomology – University of Missouri; Ph.D. 

Environmental Toxicology – Clemson University

Experience: Thirteen years of experience working for APHIS preparing ecological risk 

assessments and providing assistance on environmental compliance. Prior experience 

before joining APHIS includes other government and private sector work regarding 

ecological risk assessments related to various environmental regulations.

Writer: Thomas C. Hall

Position: USDA-APHIS-WS, Operational Support Staff, Staff Wildlife Biologist, Fort Collins, CO

Education: BS Biology (Natural History) and BA Psychology – Fort Lewis College; MS Wildlife 

Ecology – Oklahoma State University

Experience: Special expertise in wildlife biology, identification, ecology, and damage 

management. Thirty-seven years of service in APHIS Wildlife Services including operations 

and research in CO for research and OR, GU, CA, OK, and NV for operations conducting a 

wide variety of programs including bird damage research and management, livestock 

protection, invasive species management, wildlife hazard management at airports, property 

and natural resource protection including waterfowl, brown tree snake, feral swine, rodent, 

and beaver damage management. Applied and supervised chlorophacinone use.

Writer-editor: Luciana Paz
32

Position: USDA-APHIS-Policy and Program Development (PPD), Environmental and Risk 

Analysis Services (ERAS), Write-editor, Riverdale MD

Education: BS Political Science, Minor in Journalism, Certificate in Education- University of 

Tampa

Experience: Three years of experience in writing and editing environmental analysis 

documents in compliance with the National Environmental Policy Act.

Editors/Contributors for “Use of Chlorophacinone in Wildlife Damage Management Risk 

Assessment”:

Editor: Shelagh DeLiberto 

Position: USDA-APHIS-Wildlife Services (WS), National Wildlife Research Center (NWRC), 

Wildlife Biologist, Fort Collins, CO

Education: BA Biology and Environmental Science – Ithaca College; MS Wildlife Biology –

Colorado State University 

Experience: Nineteen years of service in APHIS conducting wildlife research. Two years of 

experience in preparing categorical exclusions and environmental analyses in compliance 

with the National Environmental Policy Act. 

Editor: Emily Ruell 

Position: USDA-APHIS-WS, NWRC, Registration Specialist, Fort Collins, CO

Education: B.S. Zoology and Biological Aspects of Conservation – University of Wisconsin -

Madison; M.S. Ecology – Colorado State University (CSU); M.A. Political Science – CSU

Experience: Nine years of experience with APHIS WS NWRC preparing and reviewing 

vertebrate pesticide registration data submissions and other registration materials and 

providing pesticide regulatory guidance to WS, WS NWRC, and collaborators. Prior 

experience before joining APHIS includes seven years of conducting field and laboratory 

wildlife research at CSU, and environmental policy research for the U.S. Geological Survey.

Editor: Ryan Wimberly 

Position: USDA-APHIS-WS, Operational Support Staff, Staff Wildlife Biologist, Madison, TN

Education: BS Wildlife Management and Ecology – Northwest Missouri State University

Experience: Special expertise in wildlife biology, ecology, and damage management. 

Seventeen years of service with APHIS Wildlife Services, including operations and research, 

conducting a wide variety of programs, including bird damage research and management, 

livestock protection, invasive species management, wildlife hazard management at airports, 

property and natural resource protection. Expert in preparing environmental documents for 

WS programs to comply with the National Environmental Policy Act and the Endangered 

Species Act.

7.2 Internal Reviewers

USDA APHIS Wildlife Services

Reviewer: Katherine Horak

Position: USDA-APHIS-WS

Education: B.S. Mathematics, Biology, Northern Arizona University; Ph.D. Pharmacology and 

Toxicology, The University of Arizona

Experience: Fifteen years of experience with APHIS WS determining the risks of anticoagulant 

rodenticides to nontarget species. Expert in nontarget and ecological risk assessments 
33

Reviewer: Jon Grant 

Position: USDA-APHIS-WS, State Director, Albuquerque, NM 

Education: BS Wildlife - Univ. Wisconsin, Stevens Point 

Experience: Expertise in wildlife biology and wildlife damage management operations and 

research. Thirty years of service in APHIS Wildlife Services operational programs in WI and 

NM. Experience in a wide variety of damage management to include feral swine, livestock 

protection, bird damage, wildlife hazard management at airports. Have extensive experience 

applying and supervising the use of zinc phosphide.

Reviewer: Tom Halstead

Position: USDA-APHIS-WS, State Director Kansas

Education: BS Wildlife Management – University of Nebraska - Lincoln

Experience: Thirty three years experience with APHIS Wildlife Services in operations in 

Nebraska, Arizona, Washington and Kansas. Experienced in wildlife damage management 

techniques used for predation management, feral swine, bird damage, prairie dog 

management, and wildlife hazard management at airports. Have used and supervised the 

use of Rozol and zinc phosphide.

7.3 Peer Review

The Office of Management and Budget requires agencies to have peer review guidelines for 

scientific documents. The APHIS guidelines were followed to have "Use of Carcass Disposal in 

Wildlife Damage Management" peer reviewed. WS worked with the Association of Fish and 

Wildlife Agencies to have experts review the documents.

7.3.1 Peer Reviewers Selected by the Association of Fish and Wildlife Agencies

South Dakota Game, Fish, and Parks

Louisiana Department of Wildlife and Fisheries

Michigan Department of Natural Resources

7.3.2 Comments

1. While it is useful and appreciated that the risk assessment provided exposure information 

in non-target species from the ASPCA Animal Poison Control Center database and the 

USEPA Ecological Incident Data System, it would be beneficial to include information from 

state wildlife agencies that perform necropsies and ancillary diagnostic testing such as 

testing animals for anticoagulant residues. State veterinary diagnostic laboratories with a 

toxicology service will also have useful results that could be included in future risk 

assessments. Collectively, this will capture more information about exposure and toxicosis 

in free-ranging wildlife species.

Response: We appreciate this comment and agree that results from state veterinary 

diagnostic laboratories may be informative in terms of exposure of chlorophacinone to 

free-ranging wildlife species. However, state veterinary diagnostic laboratory testing 

results are not readily available without contacting individual laboratories for such 
34

data. In addition, the Risk Assessment covers WS use and risk associated with the 

use of chlorophacinone, not the use of the product by the general public or other 

pesticide applicators. We provide the ASPCA Animal Poison Center and USEPA 

Ecological Incident Data System exposure information as a comparison to the 

exposure due to use by WS. 

2. The WDM methods and activities are tracked in MIS, but take from Chlorophacinone is 

not always estimated. The risk assessment states that there are inherent challenges 

associated with counting the number of dead target and nontarget animals underground, 

but an estimate based on the number of acres or burrows should be recorded everytime. 

The risk assessment indicates that the quantity of bait applied is always recorded but the 

number of acres or burrows treated is generally recorded. This is an area that could be 

improved.

Response: WS records the amount of chlorophacinone and other pesticides used 

during each field application by entering the quantity of pesticide applied in the 

Management Information System (MIS), as described in Section 1.1. As also stated 

in Section 1.1, applications of Chlorophacinone are generally documented in the MIS 

system with a corresponding unit of measure (e.g., “acres treated”) to help express 

how the quantity of pesticide was applied. Estimated take numbers for this risk 

assessment were based on the average occupancy of burrows by adults and young 

(number per burrow) or were estimated based on the expected number of target 

animals taken per pound of bait applied (number per lb.) or expected density (number 

per acre). These methods likely overestimate actual take numbers because they 

assume 100% take of the target animals assumed to be present and exposed. The 

take estimates have been provided for the Risk Assessment to inform the analysis of 

the exposure and risk based on WS use of Chlorophacinone. Estimates of take are 

not required under EPA registrations for pesticides.

Comments received not requiring a response.

1. The chlorophacinone review was very thorough and detailed. The author(s) methodically 

describe bait placement techniques and their reasoning. It was obvious that WS 

personnel go to great lengths to reduce exposure and subsequent impacts to non-target 

wildlife.

2. Non-target take, and accidental exposures were expounded upon at length. It was made 

clear in most of the cases that WS personnel were not involved in inappropriate bait 

placement actions, which resulted in non-target mortalities.

3. Methods of research pertinent to lethal dosages in various species and exposure methods 

were well explained, and the dosages were presented in an easily digestible and 

understandable fashion.

4. Clearly, the methodologies used for WDM via chlorophacinone are effective and about as 

safe as any landscape-level poison can be. Stressing methodologies to reduce non-target 

and human exposure is very important, and the author(s) touch on it multiple times 

throughout the document.

5. Upon reading this document, I had no ambiguities. All assumptions and uncertainties 

were clearly stated. Additionally, the large number of references, many directly related to 
35

WDM via poisoning and the use of chlorophacinone, seemed quite adequate for the 

document.

6. This document provides a thorough review of the use of Chlorophacinone in wildlife 

damage management. We appreciate the opportunity to review this document and 

participate in the risk assessment reviews. The use pattern is clearly defined and includes 

direct hand baiting or using a tube or hose from a bait dispenser or a bait dispensing 

probe. Wildlife Services’ use of Chlorophacinone is primarily through belowground 

application. The standard operating procedures and mitigations to prevent adverse 

impacts are defined and include PPE, above ground products limited to locations out of 

reach of children, pets, domestic animals, and non-target wildlife or in tamper-resistant 

bait stations, cleaning up spilled bait, not allowing applications near water or areas where 

surface water is present, not applying poison in areas where raptors are actively feeding 

on voles, and requiring the applicator to return to the site within 1-2 day intervals (over at 

least a two week period) to collect and dispose of bait and dead or dying rodents. 

7. The review of secondary exposure of nontarget predators and scavengers is fairly 

comprehensive – likely because there is substantial field data relating to nontarget 

Chlorophacinone poisoning of wildlife. Chlorophacinone exposure in terrestrial mammals 

often results in high levels of mortality in most studies. Exposure in birds results in more 

moderate toxicity and sublethal effects such as decreased survival and reproduction. The 

report refers to numerous studies documenting that predators and scavengers may be 

attracted to areas with poisoned rodents because lethargic prey are easier to capture. The 

risk assessment included information from the USEPA Ecological Incident Data System 

documenting take of threatened and endangered species due to Chlorophacinone 

exposure, including bald and golden eagles and the San Joaquin kit fox. Fortunately, 

Wildlife Services states that they do not treat in areas where threatened and endangered 

species are likely to be impacted.

8. Assumptions and uncertainties are stated in the risk assessment, for example, 

acknowledging the exposure of nontarget animals that can access the burrow (predators 

and animals that share a burrow with the target species) but recognizing that this take is 

difficult to quantify. The risk assessment also identifies that there are uncertainties 

regarding toxicity information for all Chlorophacinone formulations, chronic and sublethal 

effects on nontarget wildlife species, effects of co-occurring toxic exposures, and 

cumulative or repeated impacts to people and animals. The selection of references in the 

risk assessment seemed appropriate.